on the rates of chronometers. 
393 
Experiments with the Chrono- 
meters B and C, when removed 
from rarefied into condensed 
air. 
Pressure. 
Mean daily 
rate of £>. 
Mean daily 
rate of C. 
~ in. 
+ 9s".5 
+ 18 ".6 
75 in. 
— 17 ". 2 
— 9" -5 
4 . It becomes now an interesting enquiry, to consider how 
far the ordinary changes in the density of the air may be 
likely to exercise an influence on the rate of a chronometer. 
The range of the mercurial column in London may, on an 
average, be estimated at 2 ^ inches ; and there can be no 
doubt but the difference produced in the density of the air by 
such a range must, if the transition be at all sudden, and the 
difference of density constant for twenty-four hours, or even 
less, be sufficiently considerable to affect the majority of chro- 
nometers. A great difference, however, appears to exist in 
this respect among time-keepers. The change of density, 
that in one machine of this kind would occasion an alteration 
or rate amounting to several seconds, in another, would 
scarcely produce any sensible effects ; and I have found, 
during the whole of these experiments, a considerable dif- 
ference in this particular between pocket and box chrono- 
meters ; the former being most readily affected by altera- 
tions of atmospheric density. 
In the following tables are recorded the results of different 
experiments, instituted with a view of determining the 
MDCCCXXIV. 3 E 
