130 ASTRONOMY. 
explained. That the flow in different places of the earth follows sometimes 
sooner, sometimes later, the culmination of the moon, is to be ascribed to 
local causes, as has been satisfactorily ascertained by means of many accu- 
rately conducted experiments. 
Not only the moon, but also the sun, exercises an attraction on the water, 
and the attractive forces of the sun and moon must co-operate at time of 
new and full moon, and act against each other in the quadratures. That at 
full moon a high tide must occur, might at first appear singular, until ex- 
plained by the fact that, on the side of the earth opposite to the moon, an 
elevation of the water necessarily occurs. Thus is explained the monthly 
period of ebb and flow. The tides occurring at the time of the syzigies in 
O and O’, are commonly called spring tides; those at the quadratures E, 
Ky’, neap tides. 
Finally, there are several causes of the yearly period of ebb and flow, but 
we shall here only mention those which depend on the varying distance of 
the sun from the earth. Higher tides will take place in the winter than in 
the summer months. Since the moon can never separate more than 30° 
trom the celestial equator, it is evident that within that terrestrial zone 
included between 30 degrees of north and south latitude, the tides must be 
greatest. This is confirmed by observation, since in the polar seas the 
entire phenomenon disappears. from what has been said, the conclusion 
is readily deducible—that the ocean alone, with the open and large seas 
in connexion with it, can have tides; for supposing the moon to be above 
the Caspian sea, for instance, then its waters will be attracted; yet, on 
account of the small extent of surface, this lunar attraction will be 
everywhere equal, so that an elevation of any particular part cannot take 
place. 
The Resistance of the Ether ; its Influence on the Motion of Comets. 
50. The apparent course of the inferior and superior planets has already 
( pl. 6, figs. 24, 25) been explained. Some attention will here be directed 
to a circumstance of great importance to the planets, and particularly to the 
comets. For a long while it was believed that the spaces between the 
heavenly bodies were absolutely empty, or that a perfect vacuum existed 
there. This supposition, however, does not seem to be confirmed, at least 
with respect to the interspace of our planetary system. It is well known 
that all bodies fall with equal velocity in a perfect vacuum: moreover, the 
denser the air, and the rarer the body moving in it, the more readily the 
latter loses its original velocity, since it must experience a greater resistance 
than another body of greater density moving in the same medium. This 
must be true with regard to the planets. As these have shown no diminu- 
tion in their velocity produced by the resistance, we must suppose one of 
two things: either that the interspaces of our planetary system are abso- 
lutely empty, or that if a medium really exists, it is much too rare, in 
comparison with the density of the planets, to produce any retarding 
130 
