i j6 Count Rumfqrd’s Enquiry, concerning the Nature of Heat f 
be, in their vibrations ; and the more, of course, will the volume 
of the body they compose be expanded. 
It is well known, that the pulsations occasioned in an elastic 
fluid, by the vibrations of an elastic solid body, proceed from 
that body in all directions ; and that these pulsations are every 
where (that is to say, at all distances from the body ) isochronous 
with the vibrations of the solid body ; it is known also, that 
the mean velocity of any individual particle of the fluid is less, 
in proportion as the distance of the particle is greater from the 
centre from which these pulsations proceed. 
In the case of the pulsations occasioned in the air by the vibra- 
tions of sonorous bodies, those pulsations are every where 
isochronous with the vibrations of the sonorous body; and the 
time, or frequency oi those pulsations, determines the note ; but 
it is the velocity of the particles of the air, or the breadth of the 
wave, on which the force or strength of the sound depends ; and 
this velocity becoming less, as the distance from the sonorous 
body increases, the sound is weakened in the same proportion. 
There are several circumstances which might lead us to 
suspect, that colour depends on the frequency of those pulsations 
which have been supposed to constitute light ; and that the heat 
produced by them is in proportion to their force . 
If this supposition should be well founded, a knowledge of 
that important fact might perhaps enable us to explain several 
very interesting phenomena ; — the combustion of inflammable 
bodies, for instance ; and the great intensity of the heat which 
is produced by the concentration of calorific rays. 
There are several well known experiments with burning 
glasses, which show that the intensity of the heat generated by 
