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inequalities of distance, “ the waters being pulled from the 
earth on the one side and the earth from the waters on the 
other.” 
In any of those expositions of this theory which have 
come under my notice I have failed to observe any mention 
of that resistance to the lunar attraction without the consider- 
ation of which it appears to me that the theory in question 
must be regarded as incomplete. One of the known results 
of the mutual gravitation of the earth and moon is found 
in the instance of the former body in an oscillation of its 
major portion on both sides of a given line, viz. the orbital 
path pursued by the centre of gravity of the binary system. 
The oscillation in question is certainly resisted by the 
tendency of the earth’s particles to continue their motion in 
right lines, and the amount of this resistance clearly in- 
creases with the extent or violence of the oscillation, which 
attains its maximum at that portion of the earth’s surface 
most remote from the moon, and sinks to zero at the com- 
mon centre of gravity of the system. Without these differ- 
ences of resistance to the forced oscillation caused by the 
lunar influence, it seems to me the differences of lunar attrac- 
tion would fail to produce the effects ascribed thereto. 
If this be so, there can be no doubt as to the desirability 
of making mention in any exposition of the tidal theory 
both of the resistance I have mentioned and its differences 
in extent, and the more especially as we know on the high- 
est modern astronomical authority that '‘strange difficulties” 
are experienced by many of those who attempt from the 
expositions now in use to gain a knowledge of the modus 
operandi of the tides. The tendency to continued recti- 
linear movement to which I have alluded as a resistance to 
oscillation is however well known by the name of “ cen- 
trifugal force,” which term, or any of its modern substitutes, 
may, it seems to me, be conveniently employed in consider- 
ing the theory of the tides. 
