DR. BLACK S SENSIBLE BALANCE. igf? 



Black, to James Smithson, Esq., and inserted in the Annals of Phi' 

 losrphy, N. S. X. 52. 



'' J lie apparatus I use for weighing small globules of metals, or 

 the hke, is as follows : A thin jficre of fir-wood, not thicker than 

 a shilliiii^, and a foot long, 3-lOths of an inch broad at the middle, 

 and I l-'i tenths at each end, is divided by transverse lines into 20 

 part>:. i. e. \en parts on each side of the middle. These are the 

 principal divisions, and each of them is subdivided into halves and 

 quarters. Across the middle is tixed one of the smallest needles I 

 could procure, to serve as an axis, and it is fixed in its place by 

 means of a little sealing-wax. The numerations of the divisions 

 is tVom the middle to each ernX of the beam. The fulcrum is a bit 

 of plate-brass, the niiddie of which lies Hat on my tai)le when I 

 nse the balance, and the two ends are bent up to a right angle, so 

 as to stand upright. These two ends are ground at the same tiine 

 on a flat hone, that the extreme surfaces of them may i>c in the 

 same plane ; and their distance is such that the needle, when laid 

 across them, rests on them at a small distance from the sides of 

 the beam. I'hey rise above the surface of the tal)le only one and 

 a half or two-tenths of an inch, so that the beam is very 'united in 

 its ji'ay. 



" The weights I use are one globule of gold, which weighs one 

 grain, and two or three others which weigh one-tentli of a grain 

 ea-^-h : and also a number of small rings of fine t)iass wire, made in 

 the manner first mentioned by Mr. Lewis, by appending a weight 

 to the wire, and coiling it with the tension of that weight round a 

 thicker brass wire in a close spiral, after whieh the extremity of 

 the spiral being tied hard with waxed thread, I put the covered 

 wire in a vice, and applying a sharp knife, which is struck with a 

 hammer, 1 cut through a great number of the coils at one stroke, 

 and find them as exactly equal to one another as can be desired. 

 Those T use happen to be the one-thirtieth part of a grriiii each, or 

 300 of them wei«;h ten grains ; but 1 have others much ligliter. 



"You will perceive that by menus of these weights, plac(>d on 

 different parts of the beam, I can learn the vveight of any little 

 mass, from one grain, or a little more, to the l-l'SOOth of a grain. 

 For if the thin^ to be wei'<hed weighs one grain, it will, when 

 placed on one extremity of the beam, counteri)oise the large gold 

 weJLdit at the other extremity. U it weighs half a grain, it will 

 counterpoise the heavy gold weight at five ; if it weighs 6-lOths of 

 a f^rain, you must olace the heavy gold weight at five, and one of 

 lighter ones at the extremity to counterpoise it ; and if it weighs 

 only 1, or 2, or 3, or 4-100ths of a grain, it will be counterpoised 

 bv one of the small gold weights placed at the first, or second, or 

 third, or fourth division. If, on the contrary, it weigh one grain 

 and a fraction, it will be coimterpoised by the heavy gold weight 

 at the extremity, and oiie or more of the lighter ones placed in 

 Bome oiher part of the beam. 



" This beam has served me hitherto for every purpose : but had 

 I occasion for a more dehcate one. I could make it easily by faking; 



