ENERGY. The amount of the charge of powder used to fire a rifle-bullet gives a measure of the “energy of motion” which is imparted to the bullet. To fire a bullet of double weight requires twice as much powder, because the energy of motion of a bullet, or indeed of any other moving body, is proportional to its weight. But to fire the same bullet with double speed does not merely require double the charge of powder. Four times as much powder is needed, because the energy of motion of a moving body is proportional to the _square_ of its speed. The experienced motorist is familiar with this; if our brakes stop our car in 20 feet when we are travelling 20 miles an hour, they will not stop it in 40 feet when travelling at 40 miles an hour; we need 80 feet. Double speed requires four times the distance to pull up in, because double speed represents fourfold energy of motion. In general, the energy of motion of any moving body whatever is proportional both to the weight of the body and to the square of its speed[8]. [8] This is expressed in the mathematical formula _½mv²_ for the energy of motion of a body of weight _m_ moving with a speed _v_. If _m_ is measured in grammes, and _v_ in centimetres per second, the energy of motion of the body is said to be _½mv²_ “ergs.” Thus an “erg” is the energy of motion of a body of 2 grammes weight (so that _½m_= 1) moving with a speed of one centimetre a second. As an example, the energy of an express train of 300 tons’ weight (3 × 10⁸ gms.) moving at 60 miles an hour (2682 cms. a second) is 1079 × 10¹⁴ ergs; a cannon-ball or shell weighing a ton and moving at 1520 feet a second has precisely the same energy. One of the great achievements of nineteenth-century physics was to establish the general principle known as the “conservation of energy.” Energy can exist in a number of forms, and can change about almost endlessly from one form to another, but it can never be utterly destroyed. The energy of a moving body is not lost when the body is brought to rest, it merely takes some other form. When a bullet is brought to rest by hitting a target, part of its energy of motion goes into heating up the target, and part into heating up, or perhaps even melting, the bullet. In its new guise of heat, there is just as much energy as there was in the original motion of the bullet.