Chap. III.] INERTION. 63 



tlie action of vis -inertia) is concerned, the motion 

 demonstrated mio;lit be the averao-e result of a com- 

 plication of various motions in different directions. 

 It is obvious that since we have no definite a priori 

 knowledge of such motion, we cannot do more than 

 pomt out what the different possible effects of that 

 motion might be ; and if, after determining the effects 

 of the rotation of the earth on its axis, and its mo- 

 tion in its orbit round the sun, we find in fact such 

 deviations from the effects which those motions 

 should cause as are in accordance with any one of 

 the presumable motions of the Solar System through 

 space, we shall then, from those effects, obtain a 

 knowledge of the motion which causes them. And, 

 in investigating the action of vis-inertiee, it is not 

 necessary to decide whether the motion demonstrated 

 be a simple motion of the Solar System, or the 

 average of a combination of various motions of which 

 that system: may partake. 



If the course of this as yet unknown motion of 

 the earth through space be in the plane of the 

 ecliptic, it wiU then at one period of the year be in 

 conjunction with the orbital motion of the earth, and 

 at the opposite period of the year in opposition to 

 that motion ; gradually changing in the course of 

 each year from complete conjunction to complete 

 opposition, and again from opposition to conjunction. 

 So that, apart from a knowledge of such motion, 

 effects of vis -inertia) observed in the ocean at any 



