400 Mr. Harvey on the effects of the density of air 
moment the change of pressure takes place ; or whether it 
is an effect which the air gradually produces on the machine. 
By a reference to the experiments performed with the 
box chronometer E, it would seem as if the alteration of 
pressure required two days to produce its full effect on the 
rate ; but from other experiments now about to be recorded, 
and on which I place a greater dependence, it would appear 
that the change is immediate. 
A pocket and box chronometer, possessing detached rates 
of 4- 9 ”- o, and + i". 9 , were placed under the receiver in 
air denoted in density by 2 inches of the mercurial column ; 
and which great degree of exhaustion was employed in order 
that, by producing considerable alterations of rate, the 
changes during very small intervals of time might be per- 
ceptible. 
At the expiration of an hour, the increment produced in 
the rate of the pocket chronometer by a mean of three 
observations was found to be + 1" .33; whereas the detached 
rate in the same time would have amounted only to -f- o".si, 
being a clear increase of o".96 in consequence of the dimi- 
nished pressure. At the end of the second hour the mean 
rate was found to be 4- i".23 ; and in like manner at the 
termination of the third -{- i". 35 ; of the fourth i"3o ; of 
the fifth -f 1". 10 ; of the sixth + o".8o, and at which rate it 
continued for several hours. At the end of the nineteenth 
hour the rate recovered itself and became -J- i". 28 ; at the 
twenty-second hour 4- 1 ".25, the twenty- third + o".8o, and 
at the twenty-fourth 4" i"-o 5. These different results may, 
in a practical point of view, be regarded as uniform, consi- 
dering the unavoidable errors of observation in attempting to 
