materials, under given conditions, it varies with the pressure, in a definite and measurable rate, which is spoken of as the coefficient of friction for the particular substances. It is very much greater where the two substances slide over one another than where the one rolls upon the other, as in the case of the wheel. The latter illustrates what is called rolling friction, and in practical mechanics it is used constantly to decrease the loss-as, for example, in the wheels of wagons and cars. The use of lubricants to decrease friction is equally familiar. Without them, as everyone knows, it would be impossible to run any wheel continuously upon an axle at high speed for more than a very brief period, owing to the great heat developed through friction. Friction is indeed a perpetual antagonist of the mechanician, and we shall see endless illustrations of the methods he employs to minimize its influence. On the other hand, we must recall that were it rendered absolutely nil, his machinery would all be useless. The car wheel, for example, would revolve indefinitely without stirring the train, were there absolutely no friction between it and the rail.

AVAILABLE SOURCES OF ENERGY

We have pointed out that every body whatever contains a certain store of energy, but it has equally been called to our attention that, in the main, these stores of energy are not available for practical use. There are, however, various great natural repositories of The energy upon which man is able to draw.

chief of these are, first, the muscular energy of man himself and of animals; second, the energy of air in motion; third, the energy of water in motion or at an elevation; and fourth, the molecular and atomic energies stored in coal, wood, and other combustible materials. To these we should probably add the energy of radio-active substances-a form of energy only recently discovered and not as yet available on a large scale, but which may sometime become so, when new supplies of radio-active materials have been discovered. It will be the object of succeeding chapters to point out the practical ways in which these various stores of energy are drawn upon and made to do work for man's benefit.

T

III

THE ANIMAL MACHINE

HE muscular system is not only the oldest machine in existence, but also the most

complex. Moreover, it is otherwise entitled to precedence, for even to-day, in this so-called age of steam and electricity, the muscular system remains by far the most important of all machines. In the United States alone there are some twenty million horses doing work for man; and of course no machine of any sort is ever put in motion or continues indefinitely in operation without aid supplied by human muscles. All in all, then, it is impossible to overestimate the importance of this muscular machine which is at once the oldest and the most lasting of all systems of utilizing energy.

The physical laws that govern the animal machine are precisely similar to those that are applied to other mechanisms. All the laws that have been called to our attention must therefore be understood as applying fully to the muscular mechanism. But in addition to these the muscular system has certain laws or methods of action of its own, some of which are not very clearly understood.

The prime mystery concerning the muscle is its wonderful property of contracting. For practical purposes we may say that it has no other property; the

sole function of the muscle is to contract. It can, of course, relax, also, to make ready for another contraction, but this is the full extent of its activities. A microscopic examination of the muscle shows that it is composed of minute fibres, each of which on contraction swells up into a spindle shape. A mass of such fibres aggregated together constitutes a muscle, and every muscle is attached at either extremity, by means of a tendon, to a bone. Both extremities of a muscle are never attached to the same bone-otherwise the muscle would be absolutely useless. Usually there is only a single bone between the two ends of a muscle, but in exceptional cases there may be more. As a rule, the main body of a muscle lies along the bone to which one end of it is attached, the other end of the muscle being attached to the contiguous bone placed not far from the point. The first bone, then, serves as a fulcrum on which the second bone moves as a lever, and, as already pointed out, the familiar laws of the lever operate here as fully as in the inanimate world. But a moment's reflection will make it clear that the object effected by this mechanism is the increase of motion with relative loss of energy. In other words, the muscular force is applied to the short end of the lever, and a far greater expenditure of force is required when the muscle contracts than the power externally manifested would seem to indicate.

A moment's consideration of the mechanism of the arm, having regard to the biceps muscle which flexes the elbow, will make this clear. If a weight is held in the hand it is perhaps twelve inches from the elbow.

If, while holding the weight, you will grasp the elbow with the other hand, you will feel the point of attachment of the biceps, and discover that it does not seem to be, roughly speaking, more than about an inch from the joint. Obviously, then, if you are lifting a pound weight, the actual equivalent of energy expended by the contracting biceps must be twelve pounds. But, in the meantime, when the pound weight in your hand moves through the space of one inch, the muscle has contracted by one-twelfth of an inch; and you may sweep the weight through a distance of two feet by utilizing the two-inch contraction, which represents about the capacity of the muscle.

A similar consideration of the muscles of the legs will show how the muscular system which is susceptible of but trifling variation in size, gives to the animal great locomotive power. With the aid of a series of levers, represented by the bones of our thighs, legs, and feet, we are able to stride along, covering three or four feet at each step, while no set of the muscles that effect this propulsion varies in length by more than two or three inches. It appears, then, that the muscular system gives a marvelous illustration of capacity for storing energy in a compact form and utilizing it for the development of motion.

THE TWO TYPES OF MUSCLES

The muscles of animals and men alike are divided into two systems, one called voluntary, the other involuntary. The voluntary muscles, as their name implies, are sub