Page:Popular Science Monthly Volume 12.djvu/239

Rh Now, let us see what experiment tells us on this subject: A box has been made of proper dimensions, free within from all outside disturbance or motion of air. In this box is placed a steel frame, which moves like a gate on a very delicate hinge, so as to avoid all possible friction. A weight of nearly twenty pounds rests on this gate at about four feet from the hinge. The hinge, whose lower part is a mere point, or delicate pivot, and the weight, are in the same line, parallel with the meridian. The weight is free, as nearly as can be, to obey the power of its own inertia. In consequence of this it moves laterally once every twenty-four hours west and east, whenever the centrifugal force is increasing and decreasing.

From noon to midnight the earth's surface is moving toward that point where its motion is more rapid, and consequently it begins to feel an increasing amount of centrifugal force. This is indicated by the apparatus, for the weight, which rests on the gate, by virtue of its inertia, lags behind and makes an apparent motion westward. This motion is, of course, not real. The earth's surface moves eastward faster than the weight, and hence the weight appears to move westward. From midnight to noon the centrifugal force felt by the earth's surface diminishes, for it is then moving toward that point where its motion eastward is less rapid. This is also indicated by the apparatus, for the weight, having gradually acquired the same velocity eastward, remains stationary at midnight a very short time. But, soon after midnight, when the earth's surface begins to feel less centrifugal force, this weight, by virtue of its inertia, resists the change of motion, and therefore moves eastward as far as it moved westward before midnight.

This movement of the weight is greatest when new-moon occurs at midnight, for the earth then feels not only the centrifugal force produced by her revolution around the sun, but, in addition, that produced also by her revolution around the centre of gravity between herself and the moon.

The motion of the weight westward begins soon after mid-day, and reaches its highest acceleration at about 8 ; the motion eastward begins soon after midnight, and reaches its highest acceleration at about

I hope soon to make a new apparatus, which shall have a longer distance between the hinge and weight, and from it more marked results can be derived.

When a body moves in a curve around a centre, it feels the effect of two forces: the one, which I call centrifugal, is the impulse which puts the body in motion; the other, which I call centripetal, is the power which draws toward the centre and keeps the body from moving in a direct line. These are the only forces acting upon a body moving in a curve. The former is sometimes called tangential, but I prefer to call it centrifugal, for it is the only force which drives from