Stonsey— Of the Kinetic Theory of Gas. 3098: 
represented by the same line in the same position, would occupy 
its place between the cause and the effect; and 
3. Because the effect might have been different, while the 
physical cause remained the same—thus, if the stone lay on the 
ground, what the vicinity of the earth would have occasioned is 
stress between the stone and the ground. 
Accordingly by referring effects in nature to the operation of 
forces, we are enabled in each case to indicate with accuracy the 
intensity and direction of the effect, without having to specify 
(a) which of several possible physical causes is the one in opera- 
tion, or (b) which of the possible kinds of effect is that which is 
being produced: and this in practice is found to be an immense 
convenience. 
Such is an outline of the principles that underlie the dyna- 
mical investigation of nature, which is the form of investigation 
that penetrates most deeply into its secrets. 
The dynamical investigation of nature being the most complete 
from the physicist’s standpoint, is of course to be preferred to any 
other wherever it can be employed. Our present knowledge of 
astronomy, of rigid dynamics, of elasticity, of hydrodynamics, are 
among its great achievements. But there are other sciences in 
which we cannot penetrate so close to the origin of things, but 
which are, nevertheless, amenable to mathematical treatment 
onwards from a station less deep-seated. In these we begin with 
happily chosen equations, the truth of which we have not succeeded 
in tracing to their dynamical source in nature, but the conse- 
quences of which we can calculate and compare with what we 
observe to occur. Of this kind are the exquisite theory of light 
which was developed by M‘Cullagh, and the enormous strides 
which our knowledge of electricity has made within the last half 
century, culminating in the marvellous electro-magnetic theory 
of light. 
In all such sciences we are greatly helped by mechanical 
illustrations, which may be regarded as working models, tiat, 
though they do not in the least profess to represent the unknown 
dynamical condition which exists in nature, furnish us with an 
apparatus which operates in ways that we can both compute and 
conceive, and which produces results that, in some important 
respects and within ascertainable limits, follow laws the same as 
