miles a second. If the original velocity exceeds seven miles a second, even by the smallest amount, then the attraction of the earth will never be able to overcome it entirely. The projectile would consequently travel outwards into space, never to return to the globe from which it started, unless it should happen that other agents not hitherto contemplated should be brought into operation.
It is known that the energy possessed by a flying bullet is proportional to the square of the velocity with which it is animated. Thus if one of two equal bullets should have a speed double as great as that of the other, then the energy possessed by the more rapidly moving of the two, in consequence of its velocity, will be four times as great as that possessed by the more slowly moving body. It thus appears that a projectile launched with a velocity twelve times as great as that which can be generated by our artillery must possess a quantity of energy one hundred and forty-four times greater than would be acquired by a bullet of equal size fired from a Woolwich gun. We would, therefore, need gunpowder one hundred and forty-four times as potent as any gunpowder we now possess if we expected to shoot a cannon ball away from the earth altogether. Need it be added that the resisting power of the cannon would have to be enormously increased to withstand the stress that such an explosive would generate. In fact, it is perfectly certain that no materials known to us would be strong enough to form a weapon sufficiently tough for the purpose.
If, however, we were performing our experiments on some of the smaller globes belonging to the solar system, then a speed considerably less than that of seven miles a second would suffice to discharge the missile finally from
