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S TR Strength of each ball of the upper ftratum is perpendicularly over the distance A B may become A D. Something obliges us to Strength of Materials. centre of the equilateral triangle below, and let these be employ force to continue this compreffion; and Ý tends Materials

. connected with the balls of the under stratum by fimilar from A, or A appears to repel D. The intensity of this spiral wires. Let there be a third and a fourth, and any tendency or repulfion may be represented by another pernumber of such strata, all connected in the same manner. pendicular Dd; and, to represent the different directions It is plain that this may extend to any size and fill any of these tendencies, or the different nature of these actions, space.-- Now let this assemblage of balls be firmly contem- we may set D d on the opposite side of A B. It is in this How Bola plated by the imagination, and be supposed to shrink con- manner that the Abbé Bolcovich has represented the actions covich rez tinually in all its demenfions, till the balls

, and their distances of corpuscular forces in luis celebrated Theory of Natural dels the from each other, and the connecting wires, all vanish from Philosophy. Newton had said, that, as the great movements corpuscular the fight as discrete individual objects. All this is very con. of the solar system were regulated by forces operating at a forces. ceivable. It will now appear like a solid body, having distance and varying with the distance, so he strongly suslength, breadth, and thickness; it may be compressed, and pected (valde suspicor) that all the phenomena of cohesion, will again resume its dimensions ; it may be stretched, and with all its modifications in the different sensible forms of will again shrink; it will move away when ftruck; in short, . aggregation, and in the phenomena of chemistry and physiit will not differ in its sensible appearance from a solid elastic ology, resulted from the similar agency of forces varying body. Now when this body is in a fate of compression, with the di tance of the particles. The learned Jesuit purfor instance, it is evident that any one of the balls is at reft, in sued this thought ; and has shown, that if we suppose an consequence of the mutual balancing of the actions of all ultimate atom of matter endowed with powers of attraction the spiral wires which connect it with those around it. It and repulsion, varying, both in kind and degree, with the will greatly conduce to the full understanding of all that fol. distance, and if this force be the same in every atom, it may lows to recur to this illustration. The analogy or resem- be regulated by such a relation to the distance from the blance between the effects of this conftitution of things and neighbouring atom, that a collection of such atoms may the effects of the corpuscular forces is very great ; and have all the sensible appearances of bodies in their different wherever it obtains, we may safely draw conclusions from forms of solids, liquids, and vapours, elastic or unelastic, and what we know would be the condition of the balls in par- endowed with all the properties which we perceive, by whose

ticular circumstances to what will be the condition of a body immediate operation the phenomena of motion by impulse, By exam. of common tangible matter. We shall just give one in and all the phenomena of chemistry, and of animal and vepies

structive example, and then have done with this liypotheti getable economy, may be produced. He shows, that not-
cal body. We can suppose it of a long shape, resting on one withstanding a perfect sameness, and even a great fimplicity
point; we can suppose two weights A, B, suspended at the in this atomical constitution, there will result from this union
extremities, and the whole in equilibrio. We commonly all that unspeakable variety of form and property which
express this state of things by saying that A and B are in diversify and embellish the face of nature. We shall take
equilibrio. This is very inaccurate. A is in fact in equili- another opportunity of giving such an account of this cele-
brio with the united action of all the springs which connect brated work as it deserves. We mention it only, by the
the ball to which it is applied with the adjoining balls. by, as far as a general notion of it will be of some service
These springs are brought into action, and each is in equi- on the present occasion. For this purpose, we just observe
librio with the joint action of all the rest. Thus through that Boscovich conceives a particle of any individual species
the whole extent of the hypothetical body, the springs are of matter to consist of an unknown number of particles of
brought into action in a way and in a degree which ma- fimpler constitution ; each of which particles, in their turn,
thematica can ealily. investigate. We need not do this : it is is compounded of particles ftill more simply constituted, and
enough for our purpose that our imagination readily dif- fo on through an unknown number of orders, till we arrive
covers that some springs are stretched, others are compreffed, at the simplest possible constitution of a particle of tangible
and that a pressure is excited on the middle point of lup- matter, susceptible of length, breadth, and thickness, and
port, and the support exerts a reaction which precisely ba- necessarily consisting of four atoms of matter. And he
lances it; and the other weight is, in like manner, in iin- fhows that the more complex we suppose the constitution of
mediate equilibrio with the equivalent of the actions of all a particle, the more must the sensible qualities of the aggre.
the springs which connect the last ball with its neighbours. gate resemble the observed qualities of tangible bodies. In
Now take the analogical or resembling cafe, an oblong piece particular, he fliows how a particle may be so constituted,
of solid matter, resting on a fulcrum, and loaded with two that although it act on one other particle of the same kind
weights in equilibrio. For the actions of the connecting through a considerable interval, the interposition of a third
springs substitute the corpuscular forces, and the result will particle of the same kind may render it totally, or almost
resemble that of the hypothesis.

totally, inactive; and therefore an assemblage of such particles
Now as there is something that is at least analogous to would form such a fluid as air. All these curious inferences
a change of distance of the particles, and a concomitant are made with uncontrovertible evidence; and the greatest
change of the intensity of the connecting forces, we may encouragement is thus given to the mathematical philofon
expreís this in the same way that we are accustomed to do pher to hope, that by cautious and patient proceeding in

in fimilar cases. Let A and B (fig. 1.) represent the cen- this way, we may gradually approach to a knowledge of the CCCCLXXXIV. tres of two particles of a coherent elastic body in their laws of cohesion, that will not fhun a comparison even with

quiescent inactive state, and let us consider only the mecha- the Principia of Newton. No step can be made in this
i nical condition of B. The body may be stretched. In this investigation, but by observing with care, and generalizing

case the distance A B of the particles may become A C. In with judgment, the phenomena, which are abundantly nu-
this state there is something which makes it necessary to merous, and much more at our command than those of
employ a force to keep the particles at this distance. C has the great and sensible mations of bodies.. Following this

a tendency towards A, or we may say that A attracts C. plan, we observe,
We may represent the magnitude of this tendency of C to- 4thly, It is matter of fact, that every body has some degree Every bo-
wards A, or this attraction of A, by a line Cc perpendicur- of compressibility and dilatability ; and when the changes of preslible
lar to A C. Again, the body may be compressed, and the dimension are so moderate that the body completely recovers and dila-




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S TR Strergth of its original dimenfions on the cessation of the changing force, probably arises from the disunion of some particles, whose Strength of Materials, the extensions or compressions are sensibly proportional to action contributed to the whole or sensible effect.

the extending or compreffing forces; and therefore ihe con- compreffions we may suppose something of the same kind; ; Lawis na nealing forces are proportional to the distances of the particles for when we compress a body in one direction, it common: aure di cu. from their quiescent, neutral, or inadive positions. This seems ly bulges out in another; and in cases of very violent action yered by to have been first viewed as a law of nature by the penetra. fome particles may be disunited, whose transverse action had Ds Hooke,

of Dr Robert Hooke, one of the most eminent phi- formerly balanced part of the compressing force. For the losophers of the last century. He published a cipher, which reader will see on refie&ion, that since the compresion in he said contained the cheory of springinels and of the mo- one direction causes the body to bulge out in the transverse tions of bodies by the action of springs. It was this, ccii

It was this, ccii direction; and since this bulging out is in opposition to the inosssttu 4.- When explained in his dissertation, publish- transverse forces of attraction, it must employ fome part of ed some years after, it was ut lenfio fic vis. This is precise- the compressing force. And the common appearances are ly the propofition just now afferted as a general fact, a law in perfe& uniformity with this conception of things. When of nature.

This dissertation is full of curious observations we press a bit of dryish clay, it swells out and cracks transa
of facts in support of his assertion. In his application to versely. When a pillar of wood is overloaded, it swells out,
the motion of bodies he gives his noble discovery of the ba- and small crevices appear in the dire&ion of the fibres. After
lance-spring of a watch, which is founded on this law. The this it will not bear half of the load. This the carpenters
spring, as it is more and more coiled up, or unwound, by the call crIPPLING; and a knowledge of the circumstances which
motion of the balance, acts on it with a force proportional modify it is of great importance, and enables us to understand
to the distance of the balance from its quiescent position. some very paradoxical appearances, as will be thown byand by.
The balance therefore is acted on by an accelerating force, This partial disuniting of particles formerly colering is,
which varies in the same manner as the force of gravity act- we imagine, the chief reason why the totality of the forces
ing on a pendulum swinging in a cycloid. Its vibrations which really oppose an external strain does not increase in
therefore must be performed in equal time, whether they are the proportion of the extentions and compressions. But suf-
wide or narrow. In the same dissertation Hooke mentions ficient evidence will also be given that the forces which would
all the facts which John Bernoulli afterwards adduced in sup- connect one particle with one other particle do not augment

of Leibnitz's whimsical doctrine of the force of bodies in the accurate proportion of the change of diltance; that
in motion, or the doctrine of the vires vivæ; a doctrine which in extensions they increase more lowly, and in compressions
Hooke might justly have claimed as his own, had he not seen more rapidly
its futulity.

But there is another cause of this deviation perhaps equal-Dactility And con:

Experiments made fince the time of Hooke show that ly effe&tual with the former. Most bodies manifest some de-another firnied hy this law is strictly true in the extent to which we have li- gree of du&ility. Now what is this? The fact is, that the cause of the expe. mited it, viz. in all the changes of form which will be com- parts have taken a few arrangement, in which they again

deviatione finients of others.

pletely undone by the elasticity of the body. It is nearly cohere. Therefore, in the pássage to this new arrangement,
true to a much greater extent. James Bernoulli, in his dif- the sensible forces, which are the joint result of many cor-
fertation on the elastic curve, relates some experiments of his puscular forces, begin to respect this cew arrangement in-
own, which seem to deviate considerably from it; but on stead of the former. This must change the simple law of
clole examination they do not. The finest experiments are corpuscular force, characteristic of the particular species of
those of Coulomb, published in some late volumes of the me. matter under examination. It does not require much reflec-
moirs of the Academy of Paris. He suspended balls by wires, tion to convince us that the poffible arrangements which the
and observed their motions of oscillation, which he found particles of a body may acquire, without appearing to change
accurately corresponding with this law,

their nature, must be more numerous according as the par-
This we shall find to be a very important fact in the doc- ticles are of a more complex conftitution; and it is reason.
trine of the strergth of bodies, and we desire the reader to able to suppose that the constitution even of the most simple
make it familiar to his mind. If we apply to this our man- kind of matter that we are acquainted with is exceedingly
ner of expressing these forces by perpendicular ordinates C, complex. Our microscopes show us animals so minute, that
Dd (fig. 1.), we mull take other situations E, F, of the a heap of them must appear to the naked eye an uniforiu
particle B, and draw E e, Ff; and we must have Dd:Ff mass with a grain finer than that of the finest marble (or ra.

BD: BF, or Cc:E2 = BC:BE. In such a suppo- zor hone; and yet each of these has not only limbs, but bones, fition Fd Bce must be a ftraight line. But we shall have muscular fibres, blood vessels, fibres, and a blood consisting, abundant evidence Ly and by that this cannot be ftri&tly in all probability, of globules organised and complex like true, and that the line Bce which limits the ordinates ex.

The imagination is here lost in wonder; and noprefsing the attractive forces becomes concave towards the thing is. left us but to adore inconceirable art and wisdom, line ABE, and that the part B df is convex towards it. and to exult in the thought ihat we are the only fpectators All that can be safely concluded from the experiments hi- of this beautiful scene who can derive plealure from the view. therto made is, that to a certain extent the forces, both at- What is trodden under foot with indifference, even by the tractive and repulsive, are fenfibly proportional to the dilata- half-reasoning elephant, may be made by us the source of tions and compreffions. For,

the purest and nioft unmixed pleasure. But let us proceed 51kly, It is univerfally observed, that when the dilatations to observe, body is

have proceeded a certain length, a less addition of force is 6thly, That the forces which connect the particles of tanmuch di. fufficient to increase the dilatation in the fame degree. This gible bodies change by a change of distance, not only in de which cona

The forces lated, a (ma lado

is always obferved when the body has been so far stretched gree, but also in kind. The particle B (fig. 1.) is attracted nect the dit:on of that it takes a set, and does not completely recover its form. by A when in the fituation Cor E. It is repelled by it when particles or furce will The like may be generally observed in compressions. Most

Most at Dor F. It is not affected by it when in the fatuation B. The cangible increase its persons will recollect

, that in violently stretching an elastic reader is requested carefully to remark, that this is not an inte Change by coru, it becomes suddenly weaker, or more easily stretched. rence foundedon the authority of our mathematical figure. The a change But these phenomena do not positively prove a diminution figure is an expression (to aßift the imagination of facts in na of ditince. of the corpuscular force acting on one particle. It more ture. It requires no force to keep the particles of a body i


our own.

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Strength of their quiescent situations: brit if they are feparated by ftretch- glasses touch each other, and we shall find a very great foree Strength of

ing the body, they endeavour (pardon the figurative expref- neceffary. By Newton's experiments it appcars, that unlela Materiale
fion) to come together again. If they are brought nearer the prismatic colours begin to appear between the glaffes,
by compression, they endeavour to recede. This endeavour is they are at least oth of an inch afunder or more. Now we
manifelted by the necessity of employing force to maintain know that a very considerable force is necessary for produ-
the extension or condensation ; and we represent this by the cing thele colours, and that the more we press the glasses
different position of our lines. But this is not all: the par- together the inore rings of colours appear. It also appears
ticle B, which is repelled by A when in the fituation For from Newton's measures, that the difference of distance be.
D, is neutral when at B, and is attracted when at C or E, tween the glasses where each of thefe colours appear is about
inay be placed at such a distance A G from A greater than the 89,00oth part of an inch. We know farther, that when
AB that it shall be again repelled, or at such a distance AH we have produced the laft appearance of a greasy or pearly
that it hall again be attracted; and these alterations may be colour, and then augment the pressure, making it about a thou-
repeated again and again. This is curious and important, fand pounds on the square inch, all colours vanish, and the
and requires something more than a bare assertion for its two pieces of glafs seem to make one transparent undikin,

guishable mass. They appear now to have no air between Light al

In the article OPTICS we mentioned the most curious and them, or to be in mathematical contact. But another fact ternately valuable observations of Sir Isaac Newton, by which it ap- fhows this conclusion to be premature. The same circles

pears that light is thus alternately attracted and repelled by of colours appear in the top of a soap bubble; and as it Led. bodies. The rings of colour which appear between the ob- grows thinner at top, there appears an unreflecting {pot ject glasses of long telescopes showed, that in the small inter in the middle.

in the middle. We have the greatest probability therefore
val of socoth of an inch, there are at least an hundred such that the perfect transparency in the middle of the two glasses
changes observable, and that it is highly probable that these does not arise from their being in contact, but because the
alternations extend to a much greater distance. At one of thickness of air between them is too small in that place for the
these diltances the light actually converges towards the fo- reflection of light. Nay, Newton expressly found no reflec-
lid matter of the glass, which we express shortly, by saying tion where the thickness was žths or more of the gooooth
that it is attracted by it, and that at the next distance it de part of an inch.
clines from the glass, or is repelled by it. The same thing All this while the glasses are strongly repelling each other,
is more fimply inferred from the phenomena of light pafling for great preffure is neceffary for continuing the appearance
by the edges of knives and other opaque bodies. We refer of those colours, and they vanish in succesion as the pressure
the reader to the experiments themselves, the detail being is diminished. This vanishing of the colours is a proof
too long for this place; and we request the reader to consi- that the glasses are moving off from each other, or repel-
der them minutely and attentively, and to form distinct no. ling each other. But we can put an end to this repulsion by
tions of the inferences drawn from them. And we desire it very strong pressure, and at the same time sliding the glasses
to be remarked, that although Sir Isaac, in his discussion, on each other. We do not pretend to account for this ef.
always considers light as a set of corpuscles moving in free fect of the fliding motion ; but the fact is, that by fo doing,
fpace, and obeying the actions of external forces like any the glaffes will cohere with very great force, so that we
other matter, the particular conclusion in which we are just shall break them by any attempt to pull them asunder.
now interested does not at all depend on this notion of the It commonly happens (at least it did so with us), that in this
nature of light. Should we, with Des Cartes or Huy- sliding compression of two smooth flat plates of glass they
gens, suppose light to be the. undulation of an elastic me. scratch and mutually destroy each other's surface.
dium, the conclusion will be the same. The undulations also worth remarking, that different kinds of glass exhibit
at certain distances are disturbed by forces directed towards different properties in this respect. Flint glass will attract
the body, and at a greater distance, the disturbing forces even though a filk fibre lies double between them, and they
tend from the body.

much more readily cohere by this Niding pressure.
But the fame alternations of attraction and repulsion may Here then are two distances at which the plates of glass
alternationsbe observed between the particles of common matter. If attract each other; namely, when the lilk fibre is interposed,

we take a piece of very flat and well polifhed glass, such as and when they are forced together with this sliding motion. терullion

are made for the horizon glasses of a good Hadley's qua- And in any intermediate situation they repel cach other, abfervable drant, and if we wrap round it a fibre of filk as it comes We sec the same thing in other solid bodies. Two pieces Lead and in the par- from the cocoon, taking care that the fibre nowhere cross of lead made perfectly clean, may be made to cohere by iron. wicher be another, and then press this pretty hard on such another grinding them together in the fame manner. It is in this dies, as

piece of glass, it will lift it up and keep it suspended. The way that pretty ornaments of silver are united to iron. glase particles therefore of the one do most certainly attraet those The piece is scraped clean, and a small bit of silver like a

of the other, and this at a distance equal to the thickness of fish scale is laid on. The die which is to strike it into a
the filk fibre. This is nearly the limit; and it sometimes re- flower or nther ornament is then fet on it, and we give it a
quires a confiderable pressure to produce the effect. The smart blow, which forces the metals into contact as firm as
preffure is effectual only by comprefling the filk fibre, and if they were foldered together. It sometimes happens that
thus diminishing the distance between the glass plates. This the die adheres to the coin so that they cannot bë separa,
adhesion cannot be attributed to the pressure of the atmo ted: and it is found that this frequently happens, when
sphere, because there is nothing to hinder the air from infi- the engraving is such, that the raised figure is not complete-
nuating itself between the plates, since they are separated ly surrounded with a smooth flat ground. The probable Probable
By the filk. Belides, the experiment fucceeds equally well cause of this is curious. When the coin has a flat surface cause
ander the receiver of an air-pump.. This most valuable ex. all around, this is produced by the molt prominent part of oily the
periment was first made by Huygens, who reported it to the die. This applies to the metal

, and completely confines to the coin. the Royal Society. It is narrated in the Philosophical the air which filled the hollow of the die.

As the pressure *I'ransactions, n° 86.

goes on, the metal is fqueezed up into the hollow of the Here then is an attraction acting, like gravity, at a di- die, but there is still air compressed between them, which dance. But take away the filk fibre, and try to make the cannot escape by any passage. It is thercíore prodigiously


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Materials. Material:

condensation. This ferves to separate the die from the than the other; for when the glasses have water interposed
metal when the stroke is over. The hollow. part of the between them instead of air, it is found, that when any par-
die has not touched the metal all the while, and we may ticular colour appears, the thickness of the plate of water is
say that the impression was made by air.

If this air

to that of the plate of air which would produce the same escape by any engraving reaching through the border, they colour nearly as 3 to 4. Now, if a piece of glass be wetcohere inseparably.

ted, and exhibit no colour, and another piece of glass be fimWe have admitted that the glass plates are in contact ply laid on it, no colour will appear; but if they are strongwhen they cohere thus firmly. But we are not certain of ly preffed, the colours appear in the same manner as if the this : for if we take thefe cohering plasses, and touch glasses had air between. Also, when glass is simply wetted, them with water, it quickly insinuates itsel? between them. and the film of water is allowed to evaporate, when it is Yet they still cohere, but can now be pretty easily sepa. thus reduced to a proper thinness, the colours show them. rated.

felves in great beauty. Repuision It is owing to this repulfion, exerted through its proper These are a few of many thousand facts, by which it is Particies

the cause sphere, that certain powders swim on the surface of water, unquestionably proved that the particles of tangible matter of matter of fome boand are wetted with great difficulty.

Certain insects can are connected by forces a&ting at a distance, varying with connect dic, swin

If acting at a wiming in a

run about on the surface of water. They have brushy feet, the distance, and alternately attractive and repulsive. fluid fpe- which occupy a confiderable surface; and if their steps are we represent these forces as we have already done in fig. 1. distance. ic fically viewed with a nagnitying glass, the surface of the water by the ordinates Cc, D d, E2, Tf, &c. of a curve, it is lighter

is seen depressed all around, resembling the footsteps of a evident that this curve mult cross the axis at all those dis felves, man walking on feather-beds. This is owing to a repul- stances where the forces change from attractive to repul.

fion between the brush and the water. A common fly five, and the curve must have branches alternately abore
cannot walk in this manner on water. Its feet are wetted, and below the axis.
because they attract the water instead of repelling it. A All these alternations of attraction and repulion take
steel needle, wiped very clean, will lie on the surface of wa- place at small and insensible distances. At all fenlible distan-
ter, making an impression as a great bar would make on a ces the particles are influenced by the attraction of gravita-
feather bed; and its weight is less than that of the displaced tion; and therefore this part of the curve must be a hyper-

A'dew drop lies on the leaves of plants without bola whole equation is y = What is the form of
touching thém mathematically, as is plain from the extreme
brilliancy of the reflection at the posterior surface; nay, the curre corresponding to the smallest distance of the para
it may be sometimes observed that the drops of rain lie on ticles ? that is, what is the mutual action between the pare
the surface of water, and roll about on it like balls on a

ticles just before their coming into absolute contact? Ana-
table. Yet all these subltances can be wetted ; that is, wa- logy should lead us to suppose it to be repullion : for folio
ter can be applied to them at such distances that they at- city is the last and simpleit form of bodies with which we
tract it.

are acquainted. Fluids are more compounded, containing What we said a little ago of water infinuating itself be. fire as an effential ingredient. We should conclude that tween the glass plates without altogether destroying their this ultimate repulsion is insuperable, for the hardelt bodies. cohesion, shows that this cohesion is not the same that obtains

are the molt elastic. We are fully entitled to say, that this. between the particles of one of the plates ; that is, the repelling force exceeds all that we have ever yet applied. two plates are not in the state of one continued mass. It to overcome it; nay, there are good reasons for saying is highly probable, therefore, that between these two states that this ultimate repulfion, by which the particles are kept: there is an intermediate state of repulfion, nay, perhaps many from mathematical contact, is really insuperable in its own fuch, alternated with attractive states.

nature, and that it is impossible to produce mathematical A piece of ice is elastic, for it rebounds and it rings. contact. Its particles, therefore, when compreffed, relile; and when We shall just mention one of these, which we con der Mathemastretched, contract again. The particles are therefore in the as unanswerable. Suppose two atoms, or ultimate particles ticul.conftate represented by B in figure 1. acted on by repulsive of matter A ard B. Let A be at reft, and B more

cact imponer

ир forces, if brought nearer ; and by attractive forces, if drawn it with the vclocity 2; and let us suppose that it comes into

tible. further alunder. Ice expands, like all other bodies, by heat. mathematical contad, and impels it (according to the comIt absorbs a vat quantity of fire ; which, by combining its

mon acceptation of the word). Both move with the relo-
attractions and repulfions with those of the particles of ice, city 1. This is granted by all to be the final result of the:
changes completely the law of action, without making any collision. Now the instant of time in which this commu-
fenfible change in the distance of the particles, and the ice nication happens is no part either of the duration of the
becomes water. In this new state the particles are again in folitary motion of A, for of the joint motion of A and B :
limits between attractive and repulsive forces ; fer water has It is the feparation or boundary between them.

It is at
been shown, by the experiments or Canton and Zimmerman, once the end of the firft, and the beginning of the second,
to be elastic or compressible. It again expands by heat. It belonging equally to both.
again absorbs a prodigious quantity of heat, and becomes velucily 2. The diftinguishing circumstance therefore of

A nas moving with the
elastic vapour ; its particles repelling each other at all di- its mechanical kate is, that it has a determination (however
ftances yet observed. Tlie distance between the particles incomprehensible) by which it would move for ever with
of one piate of glass and chofe of another which lies on it, the relocity 2, if nothing changed it. This it has during
and is carried by it, is a distance of repulfion; for the force the whole of its folitary motion, and therefore in the lalt
which supports the upper piece is acting in oppofition to its inftant of this motion, In like manner, during the whole
weight. This distance is less than that at which it would of the joint motion, and therefore in the fuit intant of this
Sufpend it below it with a filk fibre interpoitd; for no prif- motion, the atom A has a determination by which it would
matic colours appear between them when the bik fibre is move for ever with the velocity 1. In one and the same
interpofed. But the distance at which glais attra&is water inftant, therefore, the atom A has two incompatible deter-
is much less than this, for no colours appear when glass is minations. Whatever notion we call form of this itate y



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Strength of which we call velocity, as a distinction of condition, the nothing will be got hy a halty look at it. The reader will st-ength of
Materials fame impossibility of conception or the same ablurdity oc. be particularly pleased with the facility and evidence with Material,

Nor can it be avoided in any other way than by which the ingenious author has deduced all the ordinary
saying, that this change of A's inotion is brought about principles of mechanics, and with the explanation which he
by infenfible gradations; that is, that. A and B influence has given of fluidity, and his deduction from thence of
each other precisely as they would do if a nender spring the laws of hydrotatics. No part of the treatise is more
were interposed. The reader is desired to look at what we valuable than the doctrine of the propagation of pressure
have said in the article Physics, 82.

through solid bodies. This, 'however, is but just touched The two magnets there fpoken of are good represen- on in the course of the investigation of the principles of tatives of two atoms endowed with mutual powers of re- mechanics. We shall borrow as much as will suffice for pulsion ; and the communication of motion is accomplished our present inquiry into the strength of materials; and we in both cases in precisely the same manner.

trust that our readers are not displeased with this general
If, therefore, we shall ever be so fortunate as to discover sketch of the doctrine (if it may be so called) of the co-
the law of variation of that force which connects one ATOM hefion of bodies. It is curious and important in itself, The doc.
of matter with another atom, and which is therefore charac- and is the foundation of all the knowledge we can acquire rine of co-
teristic of matter, and the ultimate fource of all its sensible of the present article. We are sorry to say that it is as
qualities, the curve whose ordinates represent the kind and yet a new subject of study; but it is a very promising one, jed.
the intensity of this atomical force will be something like and we by no means despair of seeing the whole of chemi-
that sketched in fig. 2. The first branch a n B will have itry brought by its means within the pale of mechanical
AK (perpendicular to the axis AH) for its affymptote, , science. The great and distinguishing agent in chemistry
and the last branch lmo will be to all sense a hyperbola, is heat, or fire the cause of heat ; and one of its most fin.
having AO for its affymptote ; and the ordinates I L, m M, gular effects is the conversion of bodies into elastic vapour.
&c. will be proportional to AL - AM" &c. expressing by mechanical forces : for it can be opposed or prevented

We have the clearest evidence that this is brought about
the universal gravitation of matter. It will have many by external pressure, a very familiar mechanical force. We
branches B 6C, D d'E; FfG, &c. expressing attractions, may perhaps find another mechanical force which will pre-
and alternate repulfive branches CcD, EF, Gg H, &c. vent fufion.
All these will be contained within a distance A H, which
does not exceed a very minute fraction of an inclı.

Having now made our readers familiar with the mode
The fimplest particle which can be a constituent of of action in which cohesion operates in giving strength to
plest ex-

a body having length, breadth, and thickness, must confist solid bodies, we proceed to consider the itrains to whicla
tended par of four fuch atoms, all of which combine their influence on this strength is opposed.
lists of four each atom of another such particle. It is evident that A piece of solid matter is exposed to four kinds of

the curve which expresses the forces that connect two such ítrain, pretty different in the manner of their operation.
particles must be totally different from this original curve,

may be torn asunder, as in the case of ropes, stretch-Strains to
this hylarchic principle. Supposing the last known, our ers, king-poits, tye-bcams, &c.
mathematical knowledge is quite able to discover the first ; 2. It may be crushed, as in the case of pillars, posts, and trength is

opposed. but when we proceed to compose a body of particles, each truss.beams. of which consists of four such particles, we may venture to 3. It may be broken across, as happens to a joist or lever say, that the compound force which connects them is al. of any kind. most beyond our search, and that the discovery of the pri- 4.


may be wrenched or twisted, as in the case of the
mary force from an accurate knowledge of the corpuscular axle of a wheel, the nail of a press, &c.
forces of this particular matter is absolutely out of our


All that we can learn is, the possibility, nay the certain- This is the simplest of all strains, and the others are in. Matter ty, of an innumerable variety of external sensible forms and deed modifications of it. To this the force of cohesion is may be

pulled qualities, by which different kinds of matter will be distin. directly opposed, with very little modification of its action guished, arising from the number, the order of composition, by any particular circumstances. and the arrangement of the subordinate particles of which When a long cylindrical or prismatic body, such as a rod a particle of this or that kind of matter is composed. All of wood or metal, or a rope, is drawn by one end, it must these varieties will take place at those small and insensible be resisted at the other, in order to bring its cohelion into diftar.ces which are between A and H, and may produce action. When it is fastened at one end, we cannot conceive all that variety which we observe in the tangible or mecha. it any other way than as equally stretched in all its parts ; nical forms of bodies, such as elasticity, ductility, hardness, for all our observations and experiments on natural bodie's softness, fluidity, vapour, and all those unseen motions or concur in showing us that the forces which connect their actions which we observe in fusion and congelation, eva- particles, in any way whatever, are equal and opposite. This poration and condensation, folution and precipitation, cry.

is called the third law of motion ; and we admit its univeritallization, vegetable and animal assimilation and secre. fality, while we affirm that it is purely experimental (fee tion, &c. &c. &c. while all bodies must be, in a certain de- Physics). Yet we have met with dissertations by perlons gree, elastic, all must gravitate, and all must be imcompene. of eminent knowledge, where propositions are maintained trable.

inconsistent with this. During the dispute about the comThis general and satisfactory resemblance between the munication of motion, some of the ablest writers have said, appearances of tangible matter and the legitimate conse. that a spring compressed or stretched at the two ends was quence of this general hypothetical property of an atom of gradually less and less compressed or stretched from the ex. matter, affords a considerable probability that such is the tremities towards the middle : but the fame writers acknow. origin of all the phenomena. We earnestly recommend to ledged the universal equality of action and reaction, which our readers a careful perusal of Boscovich's celebrated trea- is quite incompatible with this state of the spring. No such tise. A careful perusal is necessary for seeing its value ; and inequality of compression or dilatation has ever been obfer.



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