(146.) The centrifugal force of the earth’s rotation also affects detached bodies on its surface. If such bodies were not held upon the surface by the earth’s attraction, they would be immediately flung off by the whirling motion in which they participate. The centrifugal force, however, really diminishes the effects of the earth’s attraction on those bodies, or, what is the same, diminishes their weights. If the earth did not revolve on its axis, the weight of bodies in all places equally distant from the centre would be the same; but this is not so when the bodies, as they do, move round with the earth. They acquire from the centrifugal force a tendency to fly from the axis, which increases with their distance from that axis, and is therefore greater the nearer they are to the equator, and less as they approach the pole. But there is another reason why the centrifugal force is more efficient, in the opposition which it gives to gravity near the equator than near the poles. This force does not act from the centre of the earth, but is directed from the earth’s axis. It is, therefore, not directly opposed to gravity, except on the equator itself. On leaving the equator, and proceeding towards the poles, it is less and less opposed to gravity, as will be plain on inspecting _fig. 35._, where the lines P C all represent the direction of gravity, and the lines P F represent the direction of the centrifugal force. Since, then, as we proceed from the equator towards the poles, not only the amount of the centrifugal force is continually diminished, but also it acts less and less in opposition to gravity, it follows that the weights of bodies are most diminished by it at the equator, and less so towards the poles. Since bodies are commonly weighed by balancing them against other bodies of known weight, it may be asked, how the phenomena we have been just describing can be ascertained as a matter of fact? for whatever be the body against which it may be balanced, that body must suffer just as much diminution of weight as every other, and consequently, all being diminished in the same proportion, the balance will be preserved though the weights be changed. To render this effect observable, it will be necessary to compare the effects of gravity with some phenomenon which is not affected by the centrifugal force of the earth’s rotation, and which will be the same at every part of the earth. The means of accomplishing this will be explained in a subsequent chapter. CHAP. IX. THE CENTRE OF GRAVITY.