An ingenious balance constructed by Messrs Miller and Adie for Mr Jardine, is represented in Fig. 1. of Plate LII. where AA is the beam forged of one piece of steel, which, in order to give greater strength with lightness, has the form of a rhombus, whose acute angles coincide with the centres of sus pension B, B. Theends of the arms pass through the pieces C, C, which are hollow, to receive the two rings D, D, from which the scales are suspended, and terminate in slender points, which serve as indexes to point out the inclination of the balance on the ivory arches EE. The axis or centre of support, f is about 24 inches long, 1 inch deep, and three inches thick. Its under edge, worked away to form a right angle, passes through a box g in the middle of the beam, and is fixed in its place by 10 small steel screws. The edge of the centre turns on hard and highly polish ed steel plains h, from which it is lifted, when not used by Y's, which are moved up by turning the pinion I, which works in a rack within the hollow brass pillow that forms the stand for supporting the balance. Fig. 2. sliews the pieces C, C on a large scale, that the different parts may be more distinctly seen. The rings D, D, which are hardened and well polished, are just allowed to move freely between the four hardened steel points screwed through the sides of the pieces C, C. One of the end centres C is fix ed, and the other is made to move in a slit to or from it, by the four adjusting screws A.. The balance is inclosed in a mahogany case with a glass front, to the back of which is attached the brass frame L, moveable up or down by a rack and pinion M, so as to stop the scales, and thus check the vibration of the balance when in use. The cross levels N on the bottom of the pillar, are intended to adjust the plains on which the centre turns, to a true horizontal posi tion, by means of the four screws 0, that form• the feet of the mahogany case. In performing the adjust ments of the balance, we must begin by placing the edges of the three centres in a straight line as nearly as possible, by pressing down the centrefwith the two screws'', which, if lower than the centres of suspen sion, will be known by the balance preponderating to either side. In order to adjust the centre of gravity,
so as to be just under the centre of support, the rings ' must be removed from. the ends of the beam and the slider q, and moved tip as long as the beam will librate. When it is too high, either end of the beam on being depressed, will remain in that situation. We must next try if the arms have equal lengths, by replacing the rings and suspending the scales, and then putting equal weights into them. If the balance now remains in equilibrium, the adjustments are complete ; but if either side preponderates, that arm is longer than the other, and the moveable centre must be altered ac cordingly by the screws le. The previous adjustments must now be repeated, as every alteration, in a deli cate balance, in a greater or lesser degree affects all the others, so that they can 'only be attained by ap proximation. A balance of the above construction, which we have tried, was sensible to of a grain, when loaded with 10,000 grains in each scale. (y) For further information on the subject of balances, see De La Hire, Mem. Acad. Par. torn. ix. p. Roberval, Mem. Acad. Par. torn. x. p. 343. Euler, Comment. Petrop. torn. x. p. 3. Magellan, in Ro. zier's Observations, torn. ii. p. 253. tom. xvii. p. 44, '432. Ramsden, in Rozier's Observat. torn. xl. p. '432. Shuckburgh, Phil. Trans. 1798, part ii. Ni cholson's Chemistry, chap. vi. Ludlam, Phil. Trans. 'vol. lx. 1770, p. 205. Ludicke, in Gilbert's Jour nal, vol. i. p. 323. Andrews, Repertory of Arts, vol. xi. p. 16. Prony, Annoles de Chirnie, vol. xxxvi. p. 3; and Nicholson's Journal, vol. v. p.313. Guyton, Annales de Chimie, vol. xlii. p. 23. Atwood, Gil bert's Journal, vol. iv. p. 148. Dillon, Mem. de l' In stitut. Nat. torn. iv. Ferguson's Lectures, vol. ii. 385. See also HYDRODYNAMICS, MECHANICS, and SPECIFIC GRAVITY. (0)