
MECHANICAL ACTION OF HEAT. 577 
When the question, however, is confined to the relations between tempera- 
tures and quantities of heat, a more simple process may be followed, analogous to 
that which has been applied in the preceding article to the hypothesis of Mole- 
cular Collisions. 
If a mass of elastic fluid, so much rarefied that the effect of molecular attrac- 
tion is insensible, be entirely filled with vortices, eddies, or circulating currents of 
any size and figure, so that every particle moves with the common velocity m, 
then, if the planes of revolution of these eddies be uniformly distributed in all 
possible positions, it follows, from reasoning: precisely similar to that employed in 
the preceding article, that the pressure exerted by the fluid against a plane, in 
consequence of the centrifugal force of the eddies, has the following value in 
terms of gravity :— 
eae Ae, Ui eee Es) 
or two-thirds of the hydrostatic pressure due to the velocity of the eddies »; 
V being, as before, the volume occupied by unity of weight. 
It is, however, reasonable to suppose, that the motion of the particles of atomic 
atmospheres does not consist merely in circulating currents; but that those cur- 
rents are accompanied with a certain proportionate amount of vibration,—a kind 
of motion which does not produce centrifugal force. To these we have to add 
the oscillations of the atomic nuclei, in order to obtain the mechanical equivalent 
of the whole molecular motions; which is thus found to be expressed for unity 
of weight by 
we 
37° =8 BO DARBY hts OI Ne eae, 
k being a specific coefficient. Hence it follows (denoting - by N), that the ex- 
k 

pansive pressure due to molecular motions in a perfect gas, is equal to the mecha- 
nical equivalent of those motions in unity of volume multiplied by a specific 
constant 
w.2 
BE Sv Tae Mester) Belo Te, Soe, “L uerenea 
The coefficient N has to be determined by experiment; its value for atmo- 
spheric air is known to be between 0°4 and 0-41. 
In order to account for the transmission of pressure throughout the molecular 
atmospheres, it is necessary to suppose them possessed of a certain amount of 
inherent elasticity, however small, varying proportionally to density, and inde- 
pendent of heat. Let this be represented by 
h 
ra 
then 
P=(NQ+H 5 ee EET 
is the total pressure of a perfect gas. 
VOL. XX. PART Iv. ; 7R 
