Fune 2, 1870] 
NATURE 
79 
during its downward flight, if we now suppose it dislodged 
from the top of the house. In this latter case the energy 
of position with which it begins its downward flight is 
gradually reconverted into energy of actual motion, until 
at last, when the stone reaches the ground, it has the same 
amount of velocity, and, therefore, of actual energy, which 
it had at first. : 
Let us now revert, for a moment, to the definition of 
energy, which means the power of doing work, and we 
shall see at once how we may gauge numerically the 
quantity of energy which the stone possesses, and in 
order to simplify matters, let us suppose that this stone 
weighs exactly one pound. If therefore, it has velocity 
enough to carry it up one foot, it may be said to have 
energy enough to do one unit of work, inasmuch as we 
have defined one pound raised one foot high to be one 
unit of work; and in like manner if it has velocity suffi- 
cient to carry it 16 feet high, it may be said to have an 
energy equivalent to 16 units of work or foot-pounds as 
those units are sometimes called. Now, if the stone be 
discharged upwards with an initial velocity of 32 feet 
per second, it will rise 16 feet high, and it has there- 
fore an energy represented by 16, But if its initial 
velocity be 64 feet per second it will rise 64 feet high 
before it turns, and will therefore have energy repre- 
sented by 64. Hence we see that by doubling the 
velocity the energy is quadrupled, and we might show 
that by tripling the velocity the energy is increased 
nine times. ‘This is expressed in general terms by saying 
that the energy or quantity of work which a moving 
body can accomplish varies as the square of its velocity. 
This fact is well known to artillerymen, for a ball with a 
double velocity will penetrate much more than twice as 
far into an obstacle opposing its progress. 
Let us now take the stone or pound-weight having an 
initial velocity of 64 feet per second, and consider the state 
of things at the precise moment when it is 48 feet 
high. It will at that moment have an actual velocity of 
32 feet per second, which,as we have seen, will repre- 
sent 16 units of work. But it started from the ground 
with 64 units of work in it: what therefore has become of 
the difference—or 48 units? Evidently it has disappeared 
as actual energy ; but the stone, being 48 feet high, has an 
energy of position represented by 48 units ; so that at this 
precise moment of its flight its actual energy (16), plus 
its energy of position (48), are together equal to the whole 
energy with which it started (64). 
Here, then, we have no annihilation of energy, but 
merely the transformation of it from actual energy into that 
implied by position ; nor have we any creation of energy 
when the stone is on its downward flight, but merely the 
re-transformation of the energy of position into the original 
form of actual energy. 
We shall presently discuss what becomes of this actual 
energy after the stone has struck the ground ; but, in the 
meantime, we would repeat our remark how intimate 
is the analogy between the physical and the social world. 
In both cases we have actual energy and energy of posi- 
tion, the only difference being that in the social world it 
is impossible to measure energy with exactness, while in 
the mechanical world we can gatige it with the utmost 
précision. 
Proteus-like, this element energy is always changing 
its form; and hence arises the extreme difficulty of 
the subject, for we cannot easily retain a sufficient 
grasp of the ever-changing element to argue experi- 
mentally regarding it. All the varieties of physical energy 
may, however, be embraced under the two heads 
already mentioned, namely, energy of actual motion and 
of position. We have chosen the force of gravity, 
acting upon a stone shot up into the air, as our example ; 
but there are other forces besides gravity. Thus, a watch 
newly wound up is in a condition of visible advantage 
with respect to the force of the main-spring ; and as it 
continues to go it gradually loses this energy of posi- 
tion, converting it into energy of motion, A cross-bow 
bent is likewise in a position of advantage with respect to 
the spring of the bow ; and when its bolt is discharged, 
this energy of position is converted into that of motion. 
Thus again, a meteor, a railway train, a mountain torrent, 
the wind, all represent energy of actual visible motion ; 
while a head of water may be classed along with a stone 
at the top of a house as representing energy of position. 
The list which represents visible energy of motion and 
of position might be extended indefinitely ; but we must 
remember that there are also ifivisible molecular motions, 
which do not the less exist because they are invisible. 
One of the best known of these molecular energies is 
radiant light and heat—a species which can traverse 
space with the enormous velocity of 186,000 miles a 
second. 
Although itself emineritly silent and gentle in its action 
it is, nevertheless, the parent of most of the work which 
is done in the world, as we shall presently see when we 
proceed to another division of our subject. In the mean 
time we may state that radiant light and heat are stp- 
posed to consist of a certain undulatory motion traversing 
an etherial medium which pervades all space. 
Now, when this radiant energy falls upon a substance, 
part of it is absorbed, and in the process of absorption is 
converted into ordinary heat, The undulatoty motion 
which had previously traversed the thin ether of space 
has now become linked with gross palpable matter, and 
manifests itself in a motion which it produces in the par- 
ticles of this matter. The violence of this rotatory or 
vortex-like motion will thus form a measure of the heat 
which the matter contains, 
Another species of molecular energy consists of e/ectr7- 
city in motion. When an electric current is moving along 
a wire, we have therein the progress of a power moving 
like light with enormous velocity, and, like light, silent in 
its operation. Silent, we say, if it meets with no resist- 
ance, but exceedingly formidable if it be opposed ; for the 
awe-inspiring flash is not so much the electricity itself as 
the visible punishment which it has inflicted on the air 
for daring to impede its progress. Had there been a 
set of stout wires between the thunder-cloud and the earth, 
the fluid would have passed into the ground without dis- 
turbance. 
The molecular energies which we have now described 
may be imagined to represent motion of some sort not 
perceived by the outward eye; but present nevertheless to 
the eye of the understanding, they may therefore be 
compared to the energy of a body in visible motion, or 
actual energy as we have termed it. 
But we have also molecular energies which are more 
