9 
2. The eddies caused by visible motion which mixes the 
fluid up and continually brings fresh particles into contact 
with the surface. 
The first of these causes is independent of the velocity of 
the fluids if it be a gas is independent of its density, so that 
it may be said, to depend only on the nature of the fluid.^' 
The second cause, the effect of eddies, arises entirely from 
the motion of the fluid, and is proportional both to the 
density of the fluid, if gas, and the velocity with which it 
flows past the surface. 
The combined effect of these two causes may be expressed 
in a formula as follows : 
+ (I) 
where t is the difference of temperature between the surface 
and the fluid, p is the density of the fluid, v its velocity, 
and A and B constants depending on the nature of the fluid 
H being the heat transmitted per unit of surface of the sur- 
face in a unit of time. 
If therefore a fluid were forced along a fixed length of 
pipe which was maintained at a uniform temperature greater 
or less than the initial temperature of the gas, we should 
expect the folloAving results. 
1. Starting with a velocity zero, the gas would then 
acquire the same temperature as the tube. 2. As the velo- 
city increased the temperature at which the gas would 
emerge would gradually diminish, rapidly at first, but in a 
decreasing ratio until it would become sensibly constant and 
independent of the velocity. The velocity after which the 
temperature of the emerging gas would be sensibly constant 
can only be found for each particular gas by experiment; but 
it would seem reasonable to suppose that it would be the 
same as that at which the resistance offered by friction to 
the motion of the fluid would be sensibly proportional to 
the square of the velocity. It having been found both 
theoretically and by experiment that this resistance is con- 
nected with the diftusion of the gas by a formula: 
R = A^y + W^pv^, 
^ Maxwell’s Theory of Heat, Cliai). XIX. 
(II) 
