- 
J. P. Cooke—Buoyancy of the Atmosphere. 43 
ceeded that. of the weights by about 75 cubic centimeters. 
With this difference of volume we have a variation of ;3,ths 
of a milligram in weight for a difference of =\th of an inch 
of mercury in tension, or one centigrade degree in temperature. 
Hence with a difference in volume of one hundred cubic centi- 
effect in any given case. If the difference of volume amounts 
to 2,500 cubic centimeters, then a difference of =)44;th of an 
inch in the barometer, or of z4,;th of a degree in the ther- 
mometer, would cause a variation of ;,th of a milligram 
in the weight. So also a variation in the intensity of gravity 
amounting to only sy4,,5th of the whole amount would pro- 
duce a similar effect, and a sensible variation would follow any 
marked change in the purity of the air. Hence the balance 
might be used to detect exceedingly minute changes in any one 
of these variables, provided the others could be exactly con- 
trolled; and, although, with our better methods, these applica- 
tions of the balance may be of no practical value; yet the con- 
siderations, here adduced, will serve to show how sensitive the 
instrument is to the slightest changes in the density of the 
atmosphere when loaded with vessels of large volume. The 
best method of controlling the weight in such cases is that 
adopted by Regnault in his classical work on the density of 
suspended from the pans by means of platinum wires, which 
: hang freely through holes in the base of the instrument. 
