of the Equinoxes. 65 
and the attraction of the sun on that side, where the greatest 
quantity of protuberant matter is, tends to bring down the 
equator into the ecliptic, or to cause the earth to revolve about 
a diameter of the equator. This attractive influence of the 
sun gradually increases a little till the summer solstice ; it 
then gradually decreases in the same degree till the autumnal 
equinox, when it vanishes. From the autumnal equinox to 
the winter solstice it again gradually increases a little ; and it 
then gradually decreases in the same degree till the vernal 
equinox, when it again vanishes. This recurrence and con- 
tinuance of action is annually repeated. 
Similar observations apply to the attraction of the moon on 
the protuberant parts of the earth. When a straight line 
drawn from her centre to that of the earth is in the plane of 
the equator, the attractive influence of the moon has no ten- 
dency to change the position of the equator, but in other 
situations, the attraction of the moon tends to bring the equator 
of the earth into the plane of the moon’s orbit, or causes the 
earth to move round a diameter of the equator. The recur- 
rences of the moon’s action on the protuberant parts of the 
earth, and the times of their continuing, are repeated every 
month. 
These effects of the sun and moon are to be considered 
separately ; and for the reasons already stated, each of the 
actions, combined with the diurnal revolution of the earth, 
may be considered as a particular case of compound rotatory 
motion. It is needless, however, after investigating the effects 
of the sun’s action, and expressing them in general formulae,, 
to go over the same steps for ascertaining those of the moon, 
MDCCCVII. K 
