8 
In reducing these results, however, so many things had 
to he taken into account and so many assumptions have 
been made that it can hardly be a matter of surprise if they 
have been misled. And there is one assumption which 
upon the face of it seems to be contrary to general experi- 
ence, this is, that the quantity of heat imparted by a 
given extent of surface to the adjacent fluid is independent 
of the motion of that fluid or of the nature of the surface 
whereas the cooling effect of a wind compared with still air 
is so evident that it must cast doubt upon the truth of any 
hypothesis which does not take it into account. 
In this paper I approach the problem in another manner 
from that in which it has been approached before. Starting 
with the laws recently discovered of the internal diffusion of 
fluids I have endeavoured to deduce from theoretical con- 
siderations the laws for the transmission of heat, and then 
verify these laws by experiment. In the latter respect I 
can only offer a few preliminary results ; which, however, 
seem to agree so well with general experience, as to warrant 
a further investigation of the subject, to promote which is 
my object in bringing it forward in the present incom- 
plete form. 
The heat carried off by air or any fluid from a surface, 
apart from the effect of radiation, is proportional to the 
internal diffusion of the fluid at and near the surface, i.e., is 
proportional to the rate at which particles or molecules pass 
backwards and forwards from the surface to any given 
depth within the fluid, thus, if AB be the surface and ah an 
ideal line in the fluid parallel to AB then the heat carried 
off from the surface in a given time will be proportional to 
the number of molecules which in that time pass from ah 
to AB — that is for a given difference of temperature between 
the fluid and the surface. 
This assumption is fundamental to what I have to say, 
and is based on the molecular theory of fluids. 
Now the rate of this diffusion has been shown from 
various considerations to depend on two things : — 
1. The natural internal diffusion of the fluid when at rest. 
^ Traite cle la Ohaleur, Peclet, Vol. I., p. 383. 
