34 
spencer’s ‘‘ FIRST PRINCIPLES.” 
Feb., 1889 . 
manifested only in the pull exerted by the pendulum on the 
hand. That to which we assign continuity we term energy— 
kinetic when it shows itself as motion, potential when it is 
only inferred to exist from the position of the body and the 
knowledge of the work it will do. We may use as a symbol, 
to enable us to think of this potential energy, the energy of 
a stretched indiarubber cord. If a boy projects a ball attached 
to such a cord, the ball gradually loses its motion but the 
cord stretches, and in this state of stretch we suppose it to 
possess the energy previously in the ball. If we think of 
some invisible connecting machinery between the earth and 
the pendulum, we may conceive this machinery as stretched 
when the pendulum rests at its highest point, and as in that 
state possessing the energy lost as energy of motion by the 
pendulum. 
So far I have closely followed Spencer’s masterly analysis 
of this example, here and there replacing his terms by those 
more commonly used by physicists, but in his succeeding 
statements I can no longer go with him. Let us examine one 
or two of these statements. He argues that the sense of 
muscular effort is the subjective symbol both for force aud for 
energy, though he recognises that in the latter case the 
feeling of effort is joined with consciousness of motion. It 
is true that when we exert mere muscular effort without 
moving our limbs, we do work and so lose energy and even 
become tired, but that is due to the particular mechanism 
employed. If we study the separate muscular fibres instead 
of the whole limb, we find that they are moving even when 
we are exerting only a dead pull or push without motion. 
And so our sense of effort probably accompanies a supply of 
energy to the muscles, and our feeling of fatigue probably 
accompanies a loss or absense of energy. The combination 
of effort with motion uses up a great deal more energy and 
leads more rapidly to fatigue, but the fatigue is of the same 
kind in both cases. In the objective world, however, force 
and energy are entirely distinct. We speak of the steam 
pressure in the boiler without confusing it with the horse 
power of the engine, one being force per square inch, the other 
energy per minute. We speak of the weight of a consignment 
of goods, and we admit the justice of the mileage rate of 
charge for its carriage by rail, one being force, and the other a 
charge proportional to energy expended in the carriage. 
The physicists, through painful experience, aware of the 
extreme importance of keeping these two ideas of force and 
energy distinct, or rather of recognising that the one contains 
something over and above that which the other does, are 
