J. P. Cooke—Buoyancy of the Atmosphere. 39 
servations; and since the calculations are somewhat complex, 
and the required data not always readily obtained, the formulas 
are seldom applied unless the volume of the load is quite large. 
Moreover in these formulas the effect of each factor can not 
weight. 
It is assumed that the air of the balance-case is dry ; and with 
one of Becker’s balances I have not been able to trace any ° 
effect on the weight of a glass vessel from variations of hygro- 
metric condition when two open dishes of sulphuric acid (three 
Inches in diameter) were kept in the case, which has a volume 
of about 37 cubic decimeters. Under such conditions the only 
causes which sensibly modify the weight of a small glass vessel 
(like a closed potash bulb-tube) are the variations of tempera- 
ture and pressure. The relative effect of these two variables 
will appear from the following considerations, which suggested 
the method I am to describe. 
If we assume thirty inches as the standard of barometric 
pressure it is obvious that the variation of each tenth of an 
Inch from this standard will determine a change of ,i,th in 
one inch. On the other hand the balances in our chemical 
laboratories are liable to rapid variations of temperature which 
Often exceed twenty degrees, the equivalent of two inches. 
Hence of the two variables the temperature is by far the more 
important. 
f we select the two standards of temperature and tension 
