It is obvious, that the accuracy of this result de pends on the correctness of the number 2.5, which is assumed as the average density of the hill. With the view of ascertaining the real density of Shehallicn, a complete mineralogical survey of it has been recently made by Professor Playfair. He found that it .con sisted of granular quartz, whose average density was 2.61., and of mica slate, whose average density was 2.81 ; and that the density of a homogeneous moun tain, that would have produced the same effect upon the plumb-line, was 2.716. Mr Playfair has, with great labour, computed the correction that must be made on the attraction of the mountain, in conse quence of the variation ,in the specific gravity of its parts ; and it would appear from these calculations, that the earth's density is about 4.867, a result which approaches nearer than the former to the result of Mr Cavendish's experiments on the attraction of leaden balls. A detailed account of Mr Playfair's survey and calculations will, we trust, be soon given to the pub lic. To the kindness of that celebrated philosopher, the editor has been indebted for the preceding inter esting facts.
The experiments made by Mr Cavendish on the attraction of leaden balls, in order to determine the density of the earth, are so intimately connected with the attraction of mountains, that we cannot omit the present opportunity of presenting our readers with an account of the apparatus which he employed, and of the results to which he was conducted.
This ingenious and simple machine was invented for the purpose of measuring the earth's density, by the Rev. John Michell, a young and accomplished philosopher, who was carried off in early life from the scientific labours which he had so successfully be gun. It afterwards came into the hands of Mr Ca vendish, who made a few improvements on its con struction, and conducted the experiment to a sueees ful issue.
A longitudinal vertical section of the instrument is represented in Plate XLIX. Fig. 2. where GGHH is the building in which it was placed, and ABCDD CBAEFFE its case; x and x are the two balls which are suspended by the wires hz from the arm ghmh, which is itself suspended by the slender wire gl. This arm consists of a slender deal rod hmh, strengthened by a silver wire hgh, which renders it sufficiently strong to support the balls.
The case, which rests on posts 13, /3 firmly fix ed into the ground, is supported and set horizontal by four screws, two of which are seen at S, S. VT and W are the leaden weights or balls, which are sus pended from the centre-pin Pp by the copper rods Rr, PrR, and the wooden box rr. This centre pin passes through a hole in the beam HH, perpendicular ly over the centre of the instrument, and turns round in it, being prevented from falling by the plate p. MM is a pulley fastened to this pin, and Mni a cord coiled round the pulley, and passing through the end wall GG. By means of this cord the observer may turn round the pulley MM, and move the weights from one situation to the other.
It is obvious that the weights W, W conspire, by their action on the balls x, x, to turn the arm hgh in the same direction. Slips of ivory, divided into 2Oths of an inch, are placed within the case at A, A, as near to the end of the arm as possible, for,the purpose of determining its position. A small vernier scale, made
of ivory, is fixed at the end of each arm, by means of which their motion may be estimated to less than the 100th of an These divisions are viewed by means of the short telescopes T and T, through slits cut in the end of the case, and stopped with glass. They are illuminated by the lamps L and L with con vex glasses, so placed as to throw their light on the divisions, the room being in every other respect dark. By means of the wooden rod FK, with an endless screw at its extremity, the observer is enabled to turn round the support ;, to which the wire gl is fasten ed, and then to move the wire till the arm settles in the middle of the case.
Let us now suppose that the arm hgh is at rest in a known position ; then when the weights are moved, the arm will instantly be drawn aside by their at traction, but it will be made to vibrate, and its vibra tion will continue a great while. By measuring the length of these vibrations, and the time of their con tinuance, Cavendish found that the force which must be applied to each ball x, in order to draw the arm 1 one division out of its natural position, is N 818N'' being the time of a vibration in seconds ; and that'the attraction of the weight on the ball is to the attrac tion of the earth upon it as .9779 to 1, or as 1 to 8739000 D, D being the density of the earth, and each of the weights weighing 2439000 grains, or be ing equal to 10.64• spherical feet of water. The at traction of the weight upon the ball will therefore be 1 of the weight of that ball, and collie8739000D quently the attraction will be able to draw the arm . out of its natural position by 818N'N' S739000D' °r 10683D divisions ; and therefore if, on moving the weights from the midway to a new position, the arm is found to move B divisions, or if it moves 213 divisions on moving the weights from one near position to the other, it follows that the density of the earth, or D, N' is 1068313 After correcting this result as obtained .
from each experiment, Mr Cavendish obtained the following Table of densities: • of the earth is nearly 5.48, a result considerably greater than was deduced from the at traction of Shehallien.
Another method of the attraction of matter, has been suggested by the learned Dr Robi son. He supposes that a sensible effect might he produced on a long plummet, or a nice spirit level, by the immense quantity of water which is brought to Annapolis Royal in Nova Scotia twice every day by the tides, which rise above an hundred feet. " If a leaden pipe," he observes, " a few hundred feet long, were laid on the level beach at right angles with the coast, and a glass pipe set upright at each end, and the whole filled with water; the water will rise at the outer end, and sink at the end next the land as the tide rises." See Bouguer's Traite de la Figure de Terre. Phil. Trans. 1775, vol. lxv. part ii. p. 495, 500. Id. 1778, vol. lxviii. p. 689. Id. 1798,p. 469. Pringle On the Attraction of Mountains, 9to, Load. 1775 ; and Robison's Elements of Mechanical Phi losophy, vol. i. p. 339. (o)