THE TIDES. 227 



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 midniffht 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 west- 

 ward. 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 appara- 

 tus, for the weight, having gradually acquired the same velocity east- 

 ward, 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 mo- 

 tion, and therefore moves eastward as far as it moved westward be- 

 fore 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 pro- 

 duced 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 p. m. ; the motion east- 

 ward begins soon after midnight, and reaches its highest acceleration 

 at about 7 a. m. 



I hope soon to make a new apparatus, which shall have a longer 

 distance between the hinge and weight, and from it more marked re- 

 sults 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 mov- 

 ing in a direct line. These are the only forces acting upon a body 

 moving in a curve. The former is sometimes called tangential, but 1 

 prefer to call it centrifugal, for it is the only force which drives from 



