ppose a, fig. 45, to represent the Appenines, sloping towards Modena, and passing under the secondary 1 at b. Suppose that the impervious strata c, does not the side of the mountain, and that the strata, both e and below it, admit the water through them; then luid would not rise in any quantity above this stratum, ot about its edges; but the pressure being constant on wer side, because the source is elevated, the moment is pierced the water flows above it, as at w, which esents a well.

many instances, wells overflow their brims, and conto discharge water, in the manner of springs. These be springs deeply situated, which happen to be struck e well, or they may be dishes of water, confined by s, or by impervious strata, inclining towards each

Fig. 46.

w

The annexed cut, fig. 46, represents inclined strata covered with alluvial deposites. The water descending along the strata, would be lost in the adjoining valley, was it ntercepted by the dyke, d, which serves as an imperdam. The water, therefore, rises and forms springs g the inside of the dyke. Now, if a well be sunk at e water will rise to the surface of the ground, and if nclined rocks be considerably higher than the well, it overflow. If the strata form a dish, one side of which nsiderably higher than the other, the same effect will roduced.

ondon and its vicinity stand over a formation, of rathpeculiar kind, called London clay. Its direction is y horizontal, and its thickness from 100 to 500 feet. covered with alluvial deposites of various thickness;

, till the depths of the wells are various, according to
thickness of the alluvium. Until within a few years,
ost of the wells in and about London, were sunk no
eper than the surface of this clay, and its impervious na-
re is of vast importance to that great city, since the water
thus retained, and a plentiful supply is always fur-
shed by means of shallow wells. But this water,
ough limpid, is hard and impure. That, however,
ich is drawn from below the clay, is perfectly soft and
insparent; and hence all the pumps about London, which
rnish such water, are of great depth, piercing the sand
low the clay.

This water, says Mr. Conybeare, frequently rises so in-
intaneously, on passing through the clay, as not to suffer
e well digger to escape, without rising above his head.
appears to rise in different places to different heights.
hus, at Liptrap's distillery, near the Thames, it rises no
gher than the level of that river; but at Tottenham, four
iles north of London, it rises sixty feet above that level;
hile at Epping, fifteen miles north of London, the water
es to within twenty-six feet of the summit of the well,
hich is 340 feet above the level of the Thames, and
erefore 314 feet above that level. This well is 420 feet
ep, of which 200 feet were sunk through by digging, and
10 bored with an auger, four inches in diameter.
ring to this depth, no water being found, the project
as relinquished, and the well was covered over; but
the end of five months it was found that the water had
sen to within twenty-six feet of the surface, and has so
ntinued ever since. The sinking of this well was there-
e 340 feet above the level of the Thames, and eighty
et below it.

After

Another well, two miles from this, at Hunter's Hall, is 0 feet deep, but its summit is seventy higher than that Epping, and 410 feet above the level of the Thames. he water in this well stands 130 feet above its bottom, hich is sixty feet above the level of the Thames; the acal elevation of this water, therefore, is not so great as at at Epping, by fifty-four feet.

that at H
derstood
water in

its botton
the level
deep, its
bottom, 8
it rises to

These facts will be better understood by fig. 47, where marks Hunter's Hall; E Epping; T Tottenham; L Lipap's well, at Mile End. a b, is the level of the sea, as dicated by that of the Thames. It will be observed that the wells reach below the level of the Thames, except

ham is 1

and its bo

above the

All the

deriving

expected

that their

don clay

el; the de a slight

well does

at Eppin Now

a great r
nication
pierced
stand at
such case

the wate

to allow may be c

ner as a

ter in the that whi the Lond under it

at Hunters' Hall. The numbers will be chiefly unstood by the explanations already given. Thus the er in the well at Hunters' Hall, stands 130 feet from bottom, the well is 350 feet, and its mouth 410 above level of the Thames. That at Epping, is 420 feet o, its summit is 340 feet above the Thames, and its om, 80 feet below it; the water is 314 feet deep, and ses to within 26 feet of the top. The well at Tottenis 130 feet deep; its top is 70 feet above the Thames, its bottom 60 feet below it, and the water rises 60 feet ve the sea.

ll these wells being sunk below the London clay, and ving their water from the same source, it might be ected that agreeably to the general law of hydrostatics their surfaces would have a common level. The Lonclay, as we have stated, is nearly on a horizontal levthe depth of the well at Hunters' Hall, however shows ght rising of the strata there, but still the water in that does not rise so high by 54 feet, as that in the well pping.

Tow did the water which supplies these wells, exist in eat reservoir, so that a full and instantaneous commution could take place between the different points ced by the wells, then the water in them all would d at the same hydrostatic level; whereas in fact, no case exists. The strata on the contrary, which bear water, though more or less porous, are still too close llow the fluid to pass with rapidity; hence such strata - be considered as acting on the water in the same manas a series of imperfect dams. Now although the wain the present case has the same general source, being which falls on the highlands, beyond the confines of London clay formation, and percolating so as to rise

it szill

t everywhere rise to the same height when the clay is rced, but the well will only drain that which presses th the greatest force in its immediate vicinity, without ecting that which is at a distance. If there is a free 1 extensive drain in any part of such a formation, then s obvious that the water in that vicinity will rise no her than the level of the drain. Thus the water in well marked L on the cut, rises no higher than the ames, bacause that river cuts through the London clay, 1 serves as a drain to the same water bearing stratum ich supplies the other wells.

Wells situated in level countries, and in alluvial formaas, generally require to be sunk only thirty or forty t, and sometimes no more than twenty before water is nd. These are not commonly supplied by springs, but rely by the draining of the water which exists within circuit of a few yards, into a cavity. During severe ughts, many such wells fail, which shows that they are plied only by the rain which percolates from the sure, and not by deeply seated springs.

But there are some extraordinary phenomena connectwith springs which require a different explanation, if eed they can be explained at all.

There is little difficulty with respect to those springs ich rise in salt marshes, or which gush from the fis-es of rocks under the sea. The sources of these are

the distant hills; or in the strata of the vicinity, situated her than their outlets; and the presence of the sea or rsh it is plain, could not affect them, since the water m these do not penetrate their sources. This principle l also account for such springs as rise on small islands little distances from the sea shore, where they could not e been collected from the rain falling there.

sink do again i

There are however springs which arise near the tops hills, and which are so situated as to make it apparent _t their sources could not exist in the same hills, nor in se in the immediate vicinity. The water with which h are supplied, must therefore, come from the higher ls or mountains, at a distance, and passing the interning valley, rise by hydrostatic force to these outlets. any rocks are so full of fissures, as to present no diffity in supposing that considerable rivulets might run Long them, at great depths below the surface. Rocks o frequently contain large cavities, so that some rivers

k down into them and disappear for miles, when they in issue from their hiding places, and continue their urses. In limestone districts it is well known that large ities are of common occurrence. Perhaps, therefore, manner in which water is conveyed to the springs, ated as above described, may be as follows. Water, m hills at a distance, and more elevated than the ings, descend through fissures, to a cavity in the valley, ich cavity communicates with another fissure running he spring. In this manner the hydrostatic pressure m the highest hill, would overcome that from the lower e, and the water would be perpetually transferred from e to the other.

The annexed cut, fig. 48, will make this obvious. The Is a, are supposed to unite and fall into the cavity bevb, from which, the greater pressure from a, forces the ter up the hill, through a fissure, to c, where the spring

ues.

That water runs in considerable streams under the rth and among the fissures of rocks, is proved by its uing in springs, sometimes in large quantities. Dr. acculloch states, that a spring in Staffordshire, is comted to discharge more water annually, than all the falls the surrounding country; and the same, even to a eater degree, is true of that of the Sorgne, in France. A writer in Featherstonhaugh's Journal, for August, 31, p. 65, refers to a great body of water which issues om the ground, ten miles from Harrisburgh, Virginia, and hich is known under the name of "Big Spring." He ys, "it should rather be called a river, so large is the dy of water which rises suddenly from the foot of a mestone hill, and continues in a stream some yards in readth, and half a foot deep, with force sufficient to turn vo large mills immediately below."

There is a spring at Kingston, R. I., which arises from