452 
Geology as a Science and an Art. 
[August, 
take a steam-engine, and try and find out the laws of its 
working, we shall, I think, find that there are three kinds of 
science involved. In the first place we may study its mode 
of working as a whole ; that is to say, we may regard the 
steam-engine as a complicated mass of mutually dependent 
parts, and find out all we can about the working of these 
several parts under ordinary conditions, and as tending to 
the working of the engine as a whole. That is one way of 
studying it. Then, secondly, we may pay special attention 
to certain definite parts of the engine, and ascertain how 
they work, not under ordinary conditions, but under simpler 
conditions when distracting influences have been removed 
or allowed for. For example, we know the pressure of the 
steam and all the other conditions of its aCtion on the 
piston, and we find out how rapidly this would propel the 
engine if there were no such thing as friction . This kind of 
science, it will be seen, differs from the first ; its generalisa- 
tions deal, if I may so say, not with things as they are , but 
with things as they might he. But for all that it aids us im- 
mensely in the study of things as they are. Thirdly, we 
may express numerically and calculate mathematically the 
rates of working of various parts of the engine, or the rela- 
tive number of revolutions which the larger and smaller 
wheels make in the same time ; that is to say, we may apply 
certain mathematical laws of relation to certain faCts con- 
nected with our engine. But these laws of relation, which 
are worked out by the third kind of science, are altogether 
abstract, and have no necessary connection with the wheels 
or their revolutions, any more than with reapers and fields, 
or steamers and sailing ships, with the vast distances of 
stars or the minute masses of molecules, or any other pro- 
blems to which they may be applied. The third kind of 
science is of no practical use by itself, but of enormous 
practical use in its application. 
Of this third kind of science, to which Mathematics and 
Logic belong, I need say nothing here. Of the second class 
Physics and Chemistry are examples. Take as an instance 
of a scientific law of this class the well-known first law of 
motion, — that a moving body if left to itself would continue 
its motion for an indefinite time in the same direction and 
with the same velocity. But no real body that we know 
anything about is “ left to itself.” So that this is, if I may 
again be allowed the expression, a law of things as they 
might be, not of things as they are. Take, again, the law 
of pressures, — that the volume of a gas varies inversely as 
the pressure put upon it. Not only does that law require 
