W. Dennis on the Theory of the Tides. 239 
earth revolve about it; whereas, it is a revolution of the centre 
of the earth around this common centre of gravity, every point 
in the surface or elsewhere having a corresponding motion: it is 
not a revolution of the parts of the earth about a point within it 
which remains fixed, but a revolution of the earth as a mass about 
its larger orbit by the sun’s, and there is consequently a corres- 
ponding excess of the attractive force over the mean and there- 
i the former case, a disturbance of the 
equilibrium of the surface waters—a tendency in those on these 
two sides to rise to a higher level, and a consequent eaten 
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Must therefore conspire to produce the same effect which will o 
Course be an exaggerated one: hence the spring tides that are 
observed about the time of new moon. Nor is what has bee 
already stated in relation to the earth’s being held to its place b 
the sun’s attraction in any way inconsistent with this result. It 
1s still held by the sun, and with the same force, but the attrac- 
tion of the moon is added to that of the sun and it is thus not 
merely held, but actually drawn out of the course which it would 
