398 Mr. Harvey on the effects of the density of air 
sary that the alteration of density should even continue for 
twenty-four hours, since, from the change of rate being 
instantaneous (as will be proved in a subsequent page) six 
hours will be sufficient, in some cases^ to disclose it. In 
cases however where the variations of the mercurial column 
are but small, and its transition from one state to another 
marked by a gradual character, the effect on the generality 
of chronometers is scarcely, if at all perceptible. 
With a difference in the mercurial column of an inch and 
three quarters, or two inches, I have little doubt but all 
time-keepers will be influenced; and it is moreover known, 
that from a species of reaction in the atmospherical columns, 
it not unfrequently happens that the greatest depression of 
the barometer succeeds to a considerable elevation of it, and 
vice versa, so as to exhibit a difference of this kind. In the 
instance of the remarkable depression of the barometer, in 
December 1821, Mr. Howard informs us it sunk on the 
25th instant to 27.83 inches, and on the 27th remained for 
12 hours stationary at 28.07 inches ; and from which time 
to the 31st it rose to 30 inches. Now, many examples might 
be selected from the experiments recorded in the preceding 
pages, to prove that a difference of two inches in the barome- 
ter, for twelve hours, would be sufficient to produce an al- 
teration of rate ; and there can be little doubt, that had the 
rates of some good chronometers been carefully attended 
to* during this singular alteration of atmospheric density, 
variations of rate, at least equivalent to that produced by 
** I have attempted, but without success, to obtain the rates of some good chro- 
nometers during this period. 
