54 THE OCEAN". [Book II. 



one period of tlie year would appear inconsistent 

 with effects observed at any other period of the year. 

 If, on the other hand, the course of this unknown 

 motion be in the line of the poles — then, if in the 

 direction of the north pole, the action of vis-inertise 

 would appear southwards at the surface of the deep 

 and central parts of the ocean, and northwards along 

 the shores ; and if in the direction of the south pole, 

 then its action would appear northwards at the sur- 

 face of the deep and central parts of the ocean, and 

 southwards along the shores.-^ And the action of vis- 

 inertise will deviate more or less from the courses 

 here described, according as the line of motion may 

 deviate more or less from the line of the poles, or 

 from the plane of the ecliptic. 



1 It is evident that by the action here explained a considerable 

 amount of warm surface water might be transferred across the 

 equator, counterbalanced by a flow of cold water in the under 

 strata. Mr. Croll points out that if oceanic circulation were 

 caused by differences of temperature it would tend to equalise the 

 temperature of the opposite hemispheres, whereas if caused by 

 the winds it would tend to increase any existing difference. And 

 to this latter action he attributes alternate glacial epochs in the 

 northern and southern hemispheres. As a circulation caused by 

 vis-inertiae might have that same effect, the suggested glacial 

 epochs are not a vahd argument in favour of the circulation being 

 caused by the winds rather than by vis-inertise. This subject is 

 further discussed in Chapter TV, of The New Principles of Natural 

 Philosoj)hy. 



