J 825.] Dr. Black on a 'oery sehsibk Balance. 5t 



at one stroke, and find them as exactly equal to one another a^ 

 can be desired. Those I use happen to be the l-30th part of it 

 grain each, or 300 of them weigh 10 grains ; but I have others 

 much lighter. s 



You will perceive that by means of these weights placed on 

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

 mass from one grain or a little more to the t^Vo ^^, ^ grain. 

 For if the thing to be weighed weighs one grain, it will, when 

 placed on one extremity of the beam, counterpoise the large gold 

 weight at the other extremity. If it weighs half a grain, it will 

 counterpoise the heavy gold weight placed at 5, If it weigh 

 -i-Vof a grain, you must place the heavy gold weight at 5, and 

 one of the hghter ones at the extremity to counterpoise it ; and 

 if it weighs only 1, or 2, or 3, or 4-lOOths of a grain, it will be 

 counterpoised by 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 counterpoised by the 

 heavy gold weight at the extremity, and one or wore of the 

 lighter ones placed in some other part of the beam. 



This beam has served me hitherto for every purpose ; but ha3 

 t occasion for a more delicate one, I could make it easily by 

 taking a much thinner and lighter slip! of wood, and grinding 

 the needle to give it an edge. It would also be easy to make it 

 carry small scales of paper for particular purposes. 



We have no chemical news. I am employed in examiniiig 

 the Iceland waters, but have been often interrupted. I never 

 heard before of the quartz-like crystals of barytes aerata, nor of 

 the sand and new earth from New Holland. Indistinct reports 

 of new metals have reached us, but no particulars. Some fur- 

 ther account of these things from you will, therefore, be very- 

 agreeable. Dr. Hutton joins me in compliments, and wishi|^|^ 

 you all good things ; and I am. Dear Sir, hi At 



Your faithful humble servant, ^ ^ 



Joseph Bi-ack* . 



" — — w 



Note by Mt\ Smiihson. — The rings mentioned above have the 

 defect of their weight being entirely accidental ; and conse- 

 quently most times very inconvenient fractions of the grain. I 

 have found that a preferable method is to ascertain the weight 

 of a certain length of wire, and then take the length of it which 

 corresponds'to the weight wanted. If fine wire is empl6yedj_ a 

 set of small weights may be thus made with great accuracy aM 

 ease. Inconvenience from the length of the wire in the higher 

 weights is obviated by rolling it round a cylindrical body to a 

 ring, and twisting this to a cord. 



This little balance is a very valuable addition to the bloWpfpe 

 apparatus, as it enables the determination of qiiaatitie^. \^.^^ 



