NATURE 
647 
THURSDAY, APRIL 28, 1870. 
WHAT IS ENERGY 2 
ie is only of late years that the laws of motion have 
been fully comprehended. No doubt it has been known 
since the time of Newton that there can be no action 
without reaction ; or, in other words, if we define momentum 
to be the product of the mass of a moving body into ‘its 
velocity of motion, then whenever this is generated in 
one direction an equal amount is simultaneously gene- 
rated in the opposite direction, and whenever it is destroyed 
in one direction an equal amount is simultaneously de- 
stroyed in the opposite direction. Thus the recoil of a 
gun is the appropriate reaction to the forward motion of 
the bullet, and the ascent of a rocket to the downrush of 
heated gas from its orifice; and in other cases where the 
action of the principle is not so apparent, its truth has 
notwithstanding been universally admitted. 
It has, for instance, been perfectly well understood for 
the last 200 years that if a rock be detached from the top 
of a precipice 144 feet high it will reach the earth with 
the velocity of 96 feet in a second, while the earth will in 
return move up to meet it, if not with the same velocity 
yet with the same momentum. But inasmuch as the mass 
of the earth is very great compared with that of the rock, 
so the velocity of the former must be very small com- 
pared with that of the latter, in order that the momentum 
or product of mass into velocity may be the same for 
both. In fact, in this case, the velocity of the earth is 
quite insensible and may be disregarded. 
The old conception of the laws of motion was thus 
sufficient to represent what takes place when the rock is 
in the act of traversing the air to meet the earth; but, on 
the othe: hand, the true physical concomitants of the crash 
which takes place when the two bodies have come together 
were entirely ignored. They met, their momentum was 
cancelled—that was enough for the old hypothesis. 
So, when a hammer descends upon an anvil, it was con- 
sidered enough to believe that the blow was stopped by 
the anvil; or when a break was applied to a carriage-wheel 
it was enough to imagine that the momentum of the 
carriage was stopped by friction. We shall presently 
allude to the names of those distinguished men who have 
come prominently forward as the champions of a juster 
conception of things, but in the meantime let us consider 
some of those influences which served to prepare men’s 
minds for the reception of a truer hypothesis. 
_ We live in a world of work, of work from which we 
cannot possibly escape; and those of us who do not 
require to work in order to eat, must yet in some sense 
perform work in order to live. Gradually, and by very 
slow steps, the true nature of work came to be understood. 
It was seen, for instance, that it involved a much less ex- 
penditure of energy for a man to carry a pound weight 
along a level road than to carry it an equal distance up to 
the top of a mountain. 
It is not improbable that considerations of this kind 
may have led the way to a numerical estimate of work. 
Thus, if we raise a pound weight one foot high against 
the force of gravity we may call it one unit of work, in 
-which case two pounds raised one foot high or one pound 
on. We have therefore only to multiply the number of 
pounds by the vertical height in feet to which they are 
raised, and the product will represent the work done 
against gravity. The force of gravity being very nearly 
constant at the earth’s surface, and always in action, is 
a very convenient force for this purpose; but any other 
force, such as that of a spring, would do equally well to 
measure work by. Generalising, we may say, the space 
moved over against a force multiplied into the intensity of 
that force will represent the quantity of work done, So 
much for the definition of work, and it is necessary to 
know what woré is before proceeding to define Exergy. 
Now what does the word Energy really mean? In the 
first place it does not mean force. 
Two substances may have an intense mutual attraction, 
in virtue of which they form a very intimate union with 
one another; but when once this union has been consum- 
mated, although the force still continues to exist, the com- 
bination is singularly deficient in Energy. Nordoes Energy 
| mean motion, for although we cannot have motion with- 
out Energy, yet we may have Energy without motion. 
By the word Energy ts meant the power of doing work; 
and the energy which a labouring man possesses means, 
in the strictly physical sense, the number of units of 
work which he is capable of accomplishing. 
This is a subject which at this stage we may attempt 
to illustrate by reference to a very different department 
of knowledge.* 
The analogy which we shall venture to institute is 
between the social and the physical world, in the hope that 
those who are more familiar with the former than with 
the latter may be led to perceive clearly what is meant 
by the word Energy in a strictly physical sense. Energy 
in the social world is well understood. Whena man pur 
sues his course, undaunted by opposition and unappalled 
by obstacles, he is said to be a very energetic man. 
By his energy is meant the power which he pos- 
sesses of overcoming obstacles ; and the amount of this 
energy is measured (in the loose way in which we measure 
such things) by the amount of obstacles which he can 
overcome—the amount of work which he can do. Such 
a man may in truth be regarded as a social cannon-ball. 
By means of his energy of character he will scatter the 
ranks of his opponents and demolish their ramparts. 
Nevertheless, a man of this kind will sometimes be 
defeated by an opponent who does not possess a tithe 
of his personal energy. Now, why is this? A reply to 
this question will, if we do not mistake, exhibit in a 
striking manner the likeness that exists between the 
social and the physical world. The reason is that, although 
his opponent may be deficient in personal energy, yet he 
may possess more than an equivalent in the high position 
which he occupies, and it is simply this position that 
enables him to combat successfully with a man of much 
greater personal energy than himself. If two men throw 
stones at one another, one of whom stands at the top 
of a house and the other at the bottom, the man at 
the top of the house has evidently the advantage. 
So, in like manner, if two men of equal personal energy 
contend together, the one who has the highest social 
* The subject has previously been discussed from this point of view by 
Messrs. “Stewart and Lockyer in an article in Macmillan’s Magazine, 
raised two feet high would represent two units, and so‘) August 1868. 
