221. =Motion in Orbits.=--Why is it, let us ask, that a cannon-ball shot horizontally from some great height will not revolve around the earth like the moon. It has the same two forces acting upon it as the moon has--viz., a projectile force, and the attraction of the earth--and both ball and moon describe a curve in their motion. But the curve of the ball bends to the earth, while that of the moon ever sweeps around the earth. Why is this? First, there is the resistance of the air continually retarding the velocity of the ball. But, secondly, even if the ball could be projected from an elevation sufficiently high to be outside of the atmosphere, the force of the projection would not be great enough. We know, from the rate of progress of the heavenly bodies in their orbits, that it would require an immense velocity to keep the ball from being brought to the earth by its attraction. The Creator of these worlds, when he launched them into their orbits, gave them precisely that impulse which is needed to balance the centripetal force of attraction, and so they pursue a middle course between the two directions in which these two forces tend to carry them. And as their velocities have never been retarded by the resistance of air or any other substance, they have been ever the same from the beginning. CHAPTER XI. THE MECHANICAL POWERS. 222. =Machines not Sources of Power.=--The Mechanical Powers, as they are termed, are six in number--viz., the Lever, the Wheel and Axle, the Inclined Plane, the Screw, and the Wedge. They are not, strictly speaking, powers; for, as you will see in the course of our investigation, they are merely means of applying power to advantage, and are not in reality sources of power. The true sources of power are the causes of motion treated of in § 181. The instrument or machine can not create power, and the only use of all the variety of tools and machinery is to enable us to _apply_ power in such a manner, with such a velocity, and in such a direction, as to effect the objects which we have in view. The term Mechanical Power, then, is not strictly proper as applied to those contrivances which commonly have this name; but the term is in so general use that it would not be well to alter it. Every instrument, however simple and insignificant, and every machine, however large or complicated, is an example of some one of the six Mechanical Powers, or of a combination of them. I will proceed to consider each of these separately. In doing this certain terms will be used which I will first explain. _Power_ is the force by which a machine or instrument is moved. _Weight_ is the resistance to be overcome. If the resistance be in some other form than that of weight it is called technically by this name. So what is called Power may be in the form of weight. The _fulcrum_ is the point on which the instrument or machine is supported while it is in motion.