12 THE OCEAN. [Book I. 



For the elucidation of the effects of the motion of 

 the earth, we have the well-known fact that when- 

 ever a vessel containing water is set in motion, the 

 water which it contains has a tendency to move along 

 its surface in the opposite direction to that in which 

 it is moved.^ 



And, since astronomical observations appear to have shown that 

 the solar system has a motion among the stars by which it is 

 carried along in the direction of the northern hemisphere, we are, 

 if those observations be reliable, compelled to the conclusion that 

 in the same manner as that in which, when the moon is inside the 

 earth's orbit, the lesser velocity of its orbital motion round the 

 earth is lost in the greater velocity of the orbital motion in which, 

 together with the earth, it is carried round the sun ; so also, in a 

 similar manner, must the velocity of the motion in which the solar 

 system is carried northwards among the stars, be lost in the 

 greater velocity of a motion in which, together with the stars, it is 

 moving southwards. 



^ The argument of the Treatise on Vis-Inertice does not allow 

 of this illustrative case being assumed to be analogous to that of the 

 earth and its ocean ; and though I have in the present work con- 

 sidered it reasonable to admit, as stated on page 4, that the oblate- 

 spheroidal shape resulting from the earth's rotation is evidence of 

 the conditions in the two cases being analogoixs, the demonstration 

 required, as stated in the following extract from the first chapter 

 of the above-mentioned treatise, is in fact given in this work 

 also ; and the Second Part of that demonstration has been given in 

 much more detail in The New Principles of Natural Philosophy : — 



* As we do not know in what manner the force which causes 

 any known motion of the earth acts upon the earth, we cannot 

 assume that by that motion any action of vis-inertise is brojight 

 into play in the ocean, but the very existence of such action 

 is necessarily a subject for demonstration, and cannot logically be 

 inferred or denied, by analogy, from any known phenomena. 



' The subject, therefore, natui'ally divides itself into two parts. 



* First, by theoretical deduction to demonstrate hypothetically 

 the action of vis-inertia; : 



