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say that what wc cannot explain by the operation of natural 
causes must be directly referred to th.Q fiat of the Author of 
Nature, and that it is presumptuous to attempt to explain it, 
is to measure His mind by our own, and to assert that where 
we are no longer able to recognise the adaptation of means to 
an end there contrivance ceases. To assume that because the 
doctrine of evolution is a useful guide in our researches 
therefore nothing more is required, is to perform a gigantic 
“ extra-pol ation 33 (to borrow a term sometimes employed in 
mathematics) ; to conclude the form of a complete curve from 
the mere infinitesimal arc which alone is open to our observation . 
The progress of science is continually bringing phenomena 
under the category of deductions from established laws, but 
at the same time it leaves barriers which it gives no indication 
that science will ever be able to get over ; nay, sometimes it 
makes the existence of such barriers more apparent. This, 
I think, is the case with the principle of the dissipation of 
energy. I will endeavour to give some idea of what this prin- 
ciple means. Imagine a condensing steam-engine at work. 
For simplicity's sake, suppose the fire removed when the 
boiler has been well heated ; make abstraction of ail the 
surroundings ; and suppose the work done by the engine to 
be that of turning round a paddle between fixed paddles, the 
fixed and the movable paddles being alike immersed in water 
belonging to the condenser. The engine would go on working 
for a time by virtue of the heat which it got from the coals 
before the fire was removed. The heat belonging to the steam 
which comes from the water in the boiler is in part conveyed 
into the condenser. I say in part, not entirely, even if we make 
abstraction of the solid materials of the engine ; for a part 
is in appearance lost, and in lieu of it we have an exact 
equivalent in the shape of work done. But in the arrange- 
ment supposed this work is converted again into heat, through 
the friction in the water in the condenser. The upshot is, 
that while in different parts of the system there is a mutual 
exchange between energy of one kind and energy of another, 
the total energy of the system remains unchanged. But 
though this be so, the system is in a very different condition 
in its initial state from what it is in its final state, when the 
temperature has become uniform throughout. At first some 
parts were hot and some were cold; and it was in consequence 
of this unequal distribution of temperature that it was possible 
to convert energy in the shape of heat into energy in the shape 
of work, work which, though in the arrangement supposed it 
was expended, wasted we may say, within the system itself, 
might have been conveyed outside by a shaft, and turned to 
