Ill Df. Black OH a very shiUbk Baianct. [July, 



ttireia, the value of the atmospheric pressure, or height of the 

 baroineter. The objections to the method in regard to accuracy 

 and convenience are, however, too serious to induce the obserVet 

 to ^dopt it as a substitute for the latter instrument. 



[, {Tolc conilnucd.) 



Article VIII. 



A 1 jet ter from Dr, Black to James Smitfison, Esq, describing a 

 very sensible Balance, 



DEAR SIR, Edinburgh, Sept. 18, 1790. 



I "II AD the pleasure to receive your letter of the 9th. Th^ 

 apparatus I use for v^^eighing small globules of metals, or the 

 liKe, is as follows : A thin piece of fir wood not thicker than a 



shilling, and a foot long, r- of an inch broad in the middle, and 



~- at €ach end, is divided by transverse lines into 20 parts; 



that is, 1 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 fixed 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 numeration of the 

 divisions is from the middle to each end of the beam. The ful>- 

 crurais a bit of plate brass, the middle of which lies flat on my 

 table when I use the balance, and the two ends are bent up to 

 a right angle so as to ^and upright* These two ends are ground 

 at the same time on a flat hone, that the extreme surfaces of 

 them may be in the same plane ; and their distance is such that 

 the needle when laid across them rests on them at a small dis- 

 tance from the' sides of the beam. They tlse above the surface 

 of the table only one and a half or two-tenths of an inph, so that 

 the beam is very limited in its play. -- * ■ . 



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

 grain; and two or three others which weigh one- tenth of a grain 

 each ; and also a number of small rings of fine brass wire made 

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

 weight to the vvire, and coiling it with the tension of that weight 

 round a thicker brass wire in a close spiral, after which 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, I cut through a great number of the^ coils 



