88 
If we discard for the time all considerations of how it has 
come to be so, and regard only the fact that the earth and 
moon together constitute a system which revolves upon its 
common centre of gravity, I am of opinion that we shall 
have unmixed with distracting matter all the facts that are 
necessary for theorising on the production of the lunar tides. 
It will be clear that the revolution in question will be 
accompanied by the development of centrifugal force, and 
the whole point to which attention is now directed is whe- 
ther or no the said centrifugal force contributes in any 
material manner to the phenomena of the tides. It is need- 
less to complicate the matter by any attempt to reduce the 
value of this centrifugal force to an expression of the motion 
or pressure of a number of tons or pounds. We know that 
at the earth’s centre it is just sufficient to overcome that 
tendency to juncture which is the simple resultant of the 
gravitation subsisting between the earth and moon. Were 
it greater, the interval of separation would increase ; whilst 
were it less, it would diminish. The constancy of this 
intervaF proves the equality at the earth’s centre at any 
rate of the attraction which perturbs and the centrifugal 
force which offers resistance to perturbation. 
For convenience of expression we may apply the term 
“ lunar unit” to indicate the quantity of either of the forces 
we have named. It will at once be evident that on the 
side of the earth nearest the moon there will be experienced 
the effect of more than a lunar unit of attraction and less 
than a lunar unit of resistance. Indeed the centrifugal 
force which at the further side of the earth is opposed to 
the attraction of the moon here augments the effects pro- 
duced thereby, so that we have a tide-producing force equal 
to the sum of the forces in question. On the other side of 
the earth, however, we have less than a lunar unit of attrac- 
tion, and more than a lunar unit—in fact more than two lunar 
* The writer is not unaware of the ellipticitj of the moon’s orbit. 
