Those ultimate elements, however, atoms and ether, only give us what may be called the dead half of the universe, which could not exist without the constant presence of the animating principle of force or energy. Energy is the term generally adopted in the language of science, for force is apt to be associated with human effort and with actual motion produced, while energy is a comprehensive term, embracing whatever produces or is capable of producing motion. Thus, if we bend a cross-bow, the force with which it is bent may either reappear at once in the flight of the arrow, if we let go the string; or it may remain stored up, if we fix the string in the notch, ready to reappear when we pull the trigger. In the former case it is called energy of motion, in the latter energy of position. It is important to realise this distinction clearly, for many of the ordered and harmonious arrangements of the universe depend on the polarity, or conflict with alternate victories and defeats, between those two forms of energy. [Illustration] Thus if A B is a pendulum suspended at the point A, if we move it from its position of rest A C to A B and hold it there, its whole energy is that of position. If we let it go it swings backwards and forwards between the positions _A B_ and _A D_, and but for the resistance of the air and the friction at the point of suspension, it would so swing for ever. But in thus swinging what happens? From A B to A C energy of motion keeps gaining on energy of position, until when the pendulum reaches C, it has annihilated it. Energy of position has entirely disappeared, and the whole original force expended in raising the pendulum to A B exactly reappears in the force or momentum of the pendulum at its lowest point. But is this victory final? By no means; energy of position having touched bottom, gathers, like Antæus, fresh vigour for the contest, and from the position A C upwards it gains ground on its adversary until when the pendulum reaches A D it is in its turn completely victorious. The same alternation between energy of motion and of position takes place in all rhythmical movements such as waves, which, whether in water, air, or ether, are propagated, as in the case of the pendulum, by particles forced out of their position of rest and oscillating between the two energies. [Illustration] Thus if waves run along an elastic wire A B, the particle P, which has been forced into the position _p_, oscillates backwards and forwards between _p_ and _q_, beginning with nothing but energy of position at _p_, losing it all for energy of motion at P, and regaining it at _q_. All wave-motions therefore—that is to say, all sound, light, and heat—depend on this primitive polarity.