408 
Mr. Harvey on the effects of density of air 
consequence of the immediate effort made by the receiver, 
and the surrounding objects, to restore the primitive tempe- 
rature. Now, although a sudden change of temperature, of 
forty or fifty degrees, if allowed to maintain its influence for 
any considerable time, would perhaps in some cases produce 
a small derangement of rate ; yet, from its continuing to act 
only for a few seconds, and producing, even on a sensible 
thermometer only a very small effect, it cannot be supposed 
that the adjustments for temperature in the balance of a 
chronometer, protected as they are by the thick case of the 
machine, can be in the least degree influenced by it. Accord- 
ingly, on introducing a very susceptible time-keeper, H, into 
an atmosphere 50° warmer than the ordinary state of the air, 
for ten seconds, no visible alteration of rate resulted ; and it 
may hence may be inferred, that the changes of temperature 
which have taken place during the preceding experiments, 
in consequence of variations in the density of the air, can 
have had no share in producing those changes of rate which 
have been perceived ; and that they resulted from alterations 
of pressure alone. 
A change of rate in a chronometer, from an alteration in 
the density of the medium in which it is placed, considered 
as a simple fact, seems demonstrated from the foregoing 
experiments. Different hypotheses may probably be ad- 
vanced respecting the cause ; but, the supposition which 
appears the most probable is, that a change takes place in 
the arc of vibration of the balance, in consequence of the 
altered density of the air, and a consequent variation in the 
rate of the timekeeper, from the imperfect isochronism of 
the balance. 
