THE TIDES. 273 



earth is dra\vn a^vay from the waters, and that any tide is produced 

 by the waters being left behind. 



How, then, can we account for tides occurring on opposite sides 

 of the earth at the same time ? Let us see. In the first place, sup- 

 pose the earth to occupy some place in space, and to be in a state of 

 perfect rest. Then suppose the sun to come into position, and the 

 earth to start on her journey of 68,000 miles an hour in her orbit 

 around the sun ; and suppose, too, that the earth rotates only once on 

 her axis during one revolution around the sun. Then will the same 

 side of her surface face the sun in every pai't of her orbit. Conse- 

 quently, there will be a solar tide perpetually at a part of her surface, 

 produced by centrifugal force, and at that part farthest from the sun. 

 Night and solar tide will reign with unceasing steadiness at that one 

 place ; but there will be no motion of these piled-up waters. There 

 they will stay, in a steady equilibrium, by the unceasing effect of cen- 

 trifugal force, in the same manner as can be illustrated by swinging a 

 hollow globe, partially filled with Avater, around the hand by means 

 of a cord, or by swinging a bucket filled with the same liquid, and 

 having for its bottom a piece of India-rubber, which bottom will bulge 

 out when the bucket is swung around a centre, in the same manner as 

 do the waters of the far side of the earth when she swings or sweeps 

 around the central sun with a velocity of 68,000 miles an hour. 



But there are always two solar tides occurring on opposite sides 

 of the earth. The above explanation accounts only for the solar tide 

 on the side of the earth farthest from the sun. How must we account 

 for the fact that there is also one on her side facing the sun and occur- 

 ring near noon? It is a well-known law of planetary motion that 

 centrifugal and centripetal forces are precisely equal. By virtue of 

 the first the earth seeks to fly from her centre of motion ; by virtue 

 of the second she has a tendency to fall into the central luminary ; 

 and everything on her surface is operated on in like manner. The 

 particles of her water, moving very easily among one another, are 

 therefore drawn readily away from her solid portions in opposite 

 directions. On the one side the bulging out is caused by centrif- 

 ugal, on the other by centripetal force. But, as these two forces 

 are nearly equal in all parts of the earth's orbit, the tide-waves on 

 opposite sides of her surface must also be nearly equal. The centrifu- 

 gal force is produced by the revolution of the earth around the sun ; 

 the centripetal force is caused by the force of gravity lodged in the 

 great central orb. 



It must not be understood, however, that the earth in her orbital 

 motion feels the effect of these two forces at her surface only. Every 

 particle of the matter composing the earth feels both a centrifugal and 

 a centripetal force w^hile this planet moves around the central orb, 

 and these two forces are precisely equal only at the centre of the 

 earth. But the matter of her surface most remote from the sun feels 



VOL. XI, 18 



