Be 
‘% 
40 A. McMayer on Weighis of Small Portions of Matter. 
off the suspending rod. The filament with attached disk was 
now hooked on the end of the rod fixed to the block, and was | 
then ready for graduation. a 
Not being able at the time to procure silver wire of sufficient — 
fineness, I substituted some very fine and long hair, taken from _ 
the head of a child; and having brought the centre of gravity | 
and centre of motion of a very sensitive analytical balance 
almost to coincide, I obtained a piece of the middle of a hair 
weighing exactly one-half milligram. This being divided into— 
five equal parts (each about one inch long) gave us tenths ot | 
illigram. One of these tenths being placed on the pith | 
pan, the glass filament was deflected a certain quantity, which © 
was marked on an are formed of bristol board, and so as to be 
almost touched by the deflected rod in its revolution about the — 
edge of the block. Another tenth was added and another divis: | 
ion obtained: and so on, until all five divisions were marked. 
The length of the divisions being about one-fourth of an inch, 
they were very readily subdivided into ten equal parts which | 
gave me immediately ;1,ths of a milligram. The weight of any © 
‘quantity of matter less than one-half milligram may be noW | 
estimated to ;}5th of a milligram by placing it on the pan and — 
observing the deflection. 4 
For the thousandths, still more care and patience is requir 
the filament being much finer and somewhat shorter, and the p 
disk smaller and as thin as possible. In order to obtain the | 
primary graduations of hundredths, one of the above pieces of 
hair equal to ;;th milligram is divided into ten equal parts, | 
which gives us weights of ;1;th milligram. The deflections 
caused by these weights, divided into ten equal parts, give 77 
of a milligram. | 
As the least breath of air interferes with the graduations and | 
es the whole instrument is protected by a glass case, the 
end ©: 
rf the weight; in those of circular section this law is sligh 
bounded patience. 
From the great simplicity of the above arrangement, it see™§ 
very strange that some person did not long ago invent it; bw 
to my knowledge, it has never been attempted. 
Baltimore, Md., Oct. 26th, 1857, 
