158 FROM NEBULA TO NEBULA 



the applied forces are not absolutely uniform the year 

 through, for not only is the earth nearer the sun 

 at perihelion than elsewhere in her orbit, but that place 

 happens also to be the position of the winter solstice, 

 when a greater ocean expanse is exposed to the sun's 

 tidal influence; besides, the earth travels faster in her 

 orbit then than at other times. For these reasons, the 

 spring days (24 hours) must be a few seconds shorter 

 than those of autumn, but whether our chronometers are 

 reliable enough to register these nuances is doubtful. 

 Before the time of Kepler the world mistakenly supposed 

 the planets to revolve at uniform speed. Why, then, may 

 not also axial velocities periodically vary? Such varia- 

 tions might easily exist without incurring observational 

 discovery, inasmuch as they would not necessarily affect 

 either the length of the average day or of the year. 



Lifting our eyes to other planets, the first they be- 

 hold is Mars, whose mass is only 1-9 that of our earth. 

 Situated as he is about half again as far as we from the 

 sun, the latter ? s attraction upon him is less than half as 

 great as it is on the earth. Now, Mars, too, rotates on 

 his axis, and his day is only slightly longer than our 

 own, all of which harmonizes admirably with our hy- 

 pothesis of tidal rotation. There can ultimately remain 

 only one conclusion, namely, that Mars is richly endowed 

 with oceans, proportionately more expansive, though 

 shallower than ours. When the first edition of this work 

 appeared in March, 1912, it was difficult to discover in 

 the books any testimony in support of such a deduction. 

 In fact, the reports were almost if not wholly the other 

 way, making the planet out to be a veritable desert. 

 During the last two oppositions, however, overwhelming 

 evidence of a contrary nature has grown apace, some of 

 which will be adduced in a later chapter. As for the 

 axial rotations of the sun and Jupiter, with their pecul- 

 iar problem of equatorial acceleration, and the rotations 

 of the other planets in general, these must be postponed 

 to a later chapter. 



