truvius, it does not appear that the Romans had one architect, fculptor, painter, or mufician.' Their principal profeffed writers on the subject of mufic are St. Auguftine, Martianus Capella, Boethius, and Caffiodorus. Their treatifes, fays the Author, are mere repetitions of what their predeceffors had faid before, and are ali only bullets of the fame caliber. They teach no part of mufic but the alphabet, nor can anything be acquired by the most intenfe ftudy of them, except despair and the head ach.' . The prefent volume is terminated by a differtation, containing fome judicious reflections upon the conftruction and use of certain particular mufical inftruments of antiquity; and by an explanation of the feveral excellent plates with which the Author has, in the moft liberal manner, embellished, as well as illuftrated, the prefent work. The manner, likewife, in which he has executed the more abftrufe and perilous part of his undertaking, cannot fail to raise the expectations of his numerous subscribers, and readers, with refpect to the remaining part of it. Indeed, the lights which he has thrown, and the flowers which he has ftrewed, on the darkest and moft barren tracts of antiquity, must make every reader of tafte impatient to rejoin company with him in his future paffage through the more ferene and cultivated regions of modern mufic. In the technical or scientific parts of the prefent volume, he is every where perfpicuous and inftructive; in confequence of the great extent of his researches, and of the judicious arrangement, and the excellent ufe which he has made, of his materials; and in the hiftorical or narrative department, he is animated and entertaining. Every page carries marks of its being the production of an original thinker; and it will, we doubt not, be confidered as a very valuable addition not only to the literature of this country, but to that of Europe. lection of antiquities published under the patronage and infpection of Sir William Hamilton, as well as in that lately published at Rome by Pafferio, must be convinced that the ancient inhabitants of Etruria were extremely attached to mufic; for every fpecies of mufical inftrument that is to be found in the remains of ancient Greek fculpture is delineated on the vafes of thefe collections; though the antiquity of fome of them is imagined to be much higher than the general ufe of the inftruments reprefented upon them was, even in Greece.' ART. ART. IV. PHILOSOPHICAL TRANSACTIONS, Vol. LXV. For the Year 1775. GENERAL PHYSICS. Article 48. A Propofal for measuring the Attraction of fome Hills in this Kingdom by Aftronomical Obfervations. By the Rev. Nevil Mafkelyne, B. D. F. R. S. and Aftronomer Royal. Article 49. An Account of Obfervations made on the Mountain Schehallien, for finding its Attraction. By the fame. HESE two Articles contain the hiftory of a late important philofophical expedition, very properly undertaken and executed under the aufpices of the Royal Society; with the intention of afcertaining, by decifive experiments, the truth of the great law of univerfal gravitation:-the bafis of that noble fyftem which the world owes to the genius and fagacity of Newton. According to the Newtonian theory, an attractive power is not only exerted between thofe large maffes of matter which conftitute the fun and planets; but likewife between all comparatively smaller bodies, and even between the smallest particles of which they are compofed. Agreeably to this hypothefis, a heavy body, which ought to gravitate or tend toward the centre of the earth, in a direction perpendicular to its furface, fuppofing the faid furface to be perfectly even and spherical, ought likewife, though in a lefs degree, to be attracted and tend towards a mountain placed on the earth's furface: so that a plumb-line, for inftance, of a quadrant, hanging in the neighbourhood of fuch a mountain, ought to be drawn from a perpendicular fituation; in confequence of the attractive power of the quantity of matter of which it is compofed, acting in a direction different from that exerted by the whole mafs of matter in the earth, and with a proportionably inferior degree of force. It will eafily be imagined,' fays the Aftronomer Royal, in the first of thefe papers, which was read before the Royal Society in the year 1772, that to find a fenfible attraction of any hill from undoubted experiment, would be a matter of no small curiofity, would greatly illuftrate the general theory of gravity, and would make the univerfal gravitation of matter palpable, if I may fo express myself, to every perfon, and fit to convince those who will yield their affent to nothing but downright experiment. Nor would its ufes end here; as it would ferve to give us a better idea of the total mass of the earth, and the proportional denfity of the matter near the furface compared with the mean denfity of the whole earth. The refult of fuch an uncommon experiment, which I fhould hope would prove fuccessful, would doubtless do honour to the nation where it was made, and the Society which executed it.' Gg 4 Though Though Sir Ifaac Newton had long ago hinted at an experiment of this kind; and had remarked that "a mountain of an hemispherical figure, three miles high and fix broad, would not, by its attraction, draw the plumb line two minutes out of the perpendicular:" yet no attempt to ascertain this matter, by actual experiment, was made till about the year 1738; when th French academicians, particularly Mefirs. Bouguer and Condamine, who were fent to Peru to measure a degree under the equator, attempted to difcover the attractive power of Chimboraço, a mountain in the province of Quito. According to their obfervations, which were however made under circumftances by no means favourable to an accurate folution of fo nice and difficult a problem, the mountain Chimboraço exerted an attraction equal to eight feconds. Though this experiment was not perhaps fufficient to prove fatisfactorily even the reality of an attraction, much lefs the precife quantity of it; yet it does not appear that any steps had been fince taken to repeat it. The Royal Society having, through the munificence of his Majefty, been enabled to undertake the execution of this delicate and important aftronomical experiment; the Aftronomer Royal was chofen to conduct it. After various inquiries, the mountain Schehallien, fituated nearly in the centre of Scotland, was pitched upon as the moft proper for the purpofe that could be found in this ifland. The obfervations were made by taking the meridian zenith distances of different fixed ftars, near the zenith, by means of a zenith sector of ten feet radius; first on the fouth, and afterwards on the north fide of the hill, the greatest length of which extended in an eaft and west direction. It is evident that if the mafs of matter in the hill exerted any fenfible attraction, it would cause the plumb-line of the sector, through which an obferver viewed a star in the meridian, to deviate from its perpendicular fituation, and would attract it contrary ways at the two ftations, thereby doubling the effect. On the fouth fide, the plummet would be drawn to the northward, by the attractive power of the hill placed to the northward of it: and on the north fide, a contrary and equal deflection of the plumbline would take place, in confequence of the attraction of the hill, now to the fouthward of it. The apparent zenith distances of the stars would be affected contrary ways; thofe being increased at the one station, which were diminished at the other: and the correfpondent quantities of the deflection of the plumbline would give the observer the fum of the two contrary attractions of the hill, acting on the plummet at the two ftations; By a very eafy calculation it is found that fuch a mountain would attract the plumb-line 1' 18" from the perpendicular. ་་་ཇ the the half of which will, of course, indicate the attractive power of the hill. After defcribing his excellent aftronomical apparatus, and relating in detail the hiftory of a part of his various operations during his aftronomical campaign, which lafted about four months; the Author gives the refult of them, from which it appears that the Jum of the two contrary attractions of the mountain Schehallien, in the two temporary obfervatories which were fucceffively fixed half way up the hill (where the effect of its attraction would be greateft) was equal to 11". 6.-From a rough computation, founded on the known law of gravitation; and on an affumption that the denfity of the hill was equal to the mean denfity of the earth; the Author finds that the attraction of the hill should amount to about the double of this quantity. From thence he infers that the denfity of the hill is only about half the mean denfity of the earth. It does not appear however that the mountain Schehallien has ever been a volcano, or is hollow; as it is extremely folid and dense, and feemingly composed of an intire rock. Having by this curious and accurate experiment fatisfactorily afcertained the attraction of matter, and its quantity in the prefent cafe, the Author proceeds to confider fome of the confe quences which may be drawn from it, relative to feveral of the moft important queftions in natural philofophy. We fhall quote, with fome abridgments, what he obferves on this fubject: 1. It appears from this experiment, that the mountain Schehallien exerts a fenfible attraction; therefore, from the rules of philofophifing, we are to conclude that every mountain, and indeed every particle of the earth, is endued with the fame property, in proportion to its quantity of matter. 2. The law of the variation of this force, in the inverfe ratio of the fquares of the diftances, as laid down by Sir Ifaac Newton, is also confirmed by this experiment. For, if the force of attraction of the hill had been only to that of the earth, as the matter in the hill to that of the earth, and had not been greatly increased by the near approach to its centre, the attraction thereof muft have been wholly infenfible. But now, by only fuppofing the mean denfity of the earth to be double to that of the hill, which feems very probable from other confiderations, the attraction of the hill will be reconciled to the general law of the variation of attraction in the inverse duplicate ratio of the distances, as deduced by Sir Ifaac Newton from the comparison of the motion of the heavenly bodies with the force of gravity at the furface of the earth; and the analogy of nature will be preserved. 5 3. We 3. We may now, therefore, be allowed to admit this law, and to acknowledge that the mean denfity of the earth is at leaft double of that at the furface, and confequently that the denfity of the internal parts of the earth is much greater than near the furface. Hence alfo, the whole quantity of matter in the earth will be at least as great again as if it had been all compofed of matter of the fame denfity with that at the furface; or will be about four or five times as great as if it were all compofed of water. This conclufion, he adds, is totally contrary to the hypothefis of fome naturalifts who fuppofe the earth to be only a great hollow fhell of matter; fupporting it felf from the property of an arch, with an immense vacuity in the midft of it. But, were that the cafe, the attraction of mountains, and even smaller inequalities in the earth's furface, would be very great, contrary to experiment, and would affect the measures of the degrees of the meridian much more than we find they do; and the variation of gravity, in different lati tudes, in going from the equator to the poles, as found by pendulums, would not be near fo regular as it has been found by experiment to be.' 4. He obferves, laftly, that as mountains are, by thefe ex periments, found capable of producing fenfible deflections of the plumb-lines of aftronomical inftruments; it becomes a matter of great importance in the menfuration of degrees in the meridian, either to chufe places where the irregular attractions of the elevated parts may be small; or where, by their fituation, they may compenfate or counteract the effects of each other. PAPERS relating to ELECTRICITY and METEOROLOGY. Article 32. An Account of the Effects of Lightning on a House which was furnished with a pointed Conductor, &c. By Richard Haffenden, Efq; &c. With Remarks by Mr. Henley. It appears from this Article that the houfe of Richard Haffenden, Efq; at Tenderden in Kent, had been damaged by a ftroke of lightning, although it had been furnished with a metallic pointed conductor. A chimney was ftruck at one end of the houfe, about 49 or 50 feet diftant from another chimney at the other end of the building, five feet above which the point of the conducting rod was elevated. The lightning paffed from the first-mentioned chimney, through an interrupted communication of lead, &c. at the top of the house, which was connected with the conductor; exhibiting marks of its paffage, in those parts where it met with refiftance, or interruption to its courfe; by splitting and breaking both the tiles and the rafters into thousands of pieces, and throwing fome of them to a great diftance. When it reached a leaden pipe which conveys the rain water to the earth, it was conducted thither without doing any further injury to the building. |