172 THE OCEAN. [Book VIL 



the atmosphere is continuous, whereas the propelling 

 force is not : but a rocket is not retarded in its course 

 (as long as it lasts) because the propelling force, as 

 well as the retarding action, is continuous ; and the 

 velocity throughout is therefore proportioned to 

 the amount by which the propelling force exceeds 

 the retarding action. If the motion of the earth be 

 of the latter nature, then, even admitting the retard- 

 ing action of vis-inertia, the velocity of the motion, 

 being evenly proportioned to the resistance, would 

 be as lasting as the propelling force, the endurance 

 of which need not necessarily be considered to be 

 limited, as in the case of the rocket. And even if 

 the nature of the forces which move the earth be 

 shown to be such that the action of vis-inertise in the 

 ocean and atmosphere must necessarily retard and 

 gradually annihilate the earth's motions : this would 

 not disprove the existence of the action of vis-mertias, 

 unless it be also shown that the motion of the earth 

 will never be retarded.^ The existence or non-exist- 

 ence of the action of vis-inertite in the ocean is a 

 question to be decided by practical investigation, and 

 not by theoretical opinion concerning its reaction on 

 the motions which produce it. 



The .question as to whether the action of vis- 



* A Paper on the Retardation of the Earth's Rotation, read 

 before the Royal Astronomical Society by the late Professor 

 Adams, since the first publication of the above, has practically 

 annulled the objection. 



