Tidal Wave on the Earth's Rotation, 289 



when raising the water. In regard to the extent of the rise at 

 the equator, occasioned by the earth's diurnal motion, it is of 

 little consequence whether the earth be uniformly covered with 

 water or not, for in this case the water remains permanently 

 raised ; but in regard to the rise occasioned by the earth's rota- 

 tion on the centre 0' the case is entirely different. Here the 

 rise only exists at the two places B and D in the form of an 

 exceedingly broad wave, fixed in relation to the position of the 

 moon ; and as the waters at these places are continually shifting 

 at_a rate of upwards of a thousand miles an hour, there must be 

 a constant rise and fall so as to maintain the tidal wave in its 

 proper place. But whatever breaks up the continuity of the 

 ocean, such as an island or a continent, must stand as an ob- 

 stacle in the way of the waters rising, and will thus prevent the 

 wave attaining anything like the height corresponding to the 

 centrifugal force. 



That which makes the waters rise at B is, as we have seen, 

 the centrifugal force occasioned by the earth's motion round 

 this centre 0'. But it is impossible that the waters of the ocean 

 can rise without a considerable portion of the force applied being 

 consumed in friction, in the rubbing of the molecules of the 

 water against each other or against the sea-shore, or against any 

 other obstacle standing in their way. And whatever is lost in 

 friction goes to produce heat, and can never again be restored to 

 its source. But what is its source ? What is the motion which 

 is thus consumed in friction ? It is simply the motion of the 

 molecules of the water round the centres and 0'; for at this 

 moment they are moving round both centres. It will be ob- 

 served that the molecules of the water situated, say at the point 

 C, are, in consequence of the earth's diurnal rotation, carried in 

 the direction of B. But as they approach to B, they are rece- 

 ding from the other centre of rotation 0', and consequently have 

 the velocity of their motion round this centre increased. This 

 motion reaches a maximum when the molecules arrive at the 

 point B. By this time the centrifugal force has become so much 

 increased as to cause them to fly off in a tangent to the surface 

 of the earth, to a certain extent, and thus the tidal wave is formed. 

 It is perfectly evident that the solid parts composing the earth's 

 surface would fly off along with the molecules of the water if 

 they could. But this they cannot do, because they are rigidly 

 fixed to the earth by the force of cohesion. The molecules of 

 the water as they recede will therefore pass over the solid parti- 

 cles of the earth, and in doing so will rub against them, and 

 thus lose a portion of their motion. But the motion lost, be it 

 observed, is the motion of revolution round the centres O and 

 0'. If the motion of the molecules of the water had been simply 

 Phil. Mac/. S. 4. Vol. 27. No. 182. April 1864. U 



