405 
on the rates of chronometers. 
In more than one instance it has happened, that the mer- 
curial column of the gauge of the air pump has been elevated 
during the intervals of comparison, by the introduction of 
air ; and the effect of which was in all cases, an alteration of 
rate in the time-keeper, dependent on the degree of change. 
For example, during the period when a pocket chronometer 
was subjected to air of a density represented by 12 inches of 
quicksilver, and when its rate was -j- io".o, the gauge be- 
came imperfect, the quicksilver rising to 18 inches soon 
after the time of comparison ; the succeeding day the rate 
was -j- 7". 5. Another chronometer under the same circum- 
stances also underwent an alteration in its rate from + 18^.0 
to + 14". 2. In another instance, the rate of the time keeper 
was -J- 1 5".o under a pressure of 4 inches ; during the next 
twenty-four hours the quicksilver rose to 12 inches, and the 
rate declined to + 12". 4. The next day the imperfection of 
the gauge still continuing, the mercury rose to 17^ inches, 
and the rate sunk to + 9" 9 • 
Sometimes a part of the air contained in the condensing 
engine, by escaping, would occasion fluctuations in the rates 
of the time keepers ; but on restoring the machine to per- 
fect order, they would immediately assume an uniform cha- 
racter. Thus, when a box chronometer was placed in air of 
uniform density, corresponding to 4 5 inches of mercury, the 
rates for three days were respectively - — 10 ".9, — 10 ". 6 , and 
— 10". 5 ; but on attempting to increase the density to 48 
inches of quicksilver, the machine disclosed proofs of imper- 
fection ; and the rates for three days were respectively 
— 1 5' 1 .0, — 1 2". 8 , and — 1 o". 1 . The machine was then repaired, 
and air introduced into it of a density denoted by 51 inches 
