84 SCIENCE PROGRESS 



of rotation of Mars was much shorter than at present, but that 

 by tidal action of the sun this period has been gradually length- 

 ened to its present value; at the same time the satellite's period 

 is supposed to be shortening and its distance from the planet 

 slowly diminishing. In the case of the moon, he considers that 

 millions of years ago our earth was rotating much more quickly 

 than at present. In contracting, a portion separated from the 

 rest, and gradually receded, becoming the moon. The earth's 

 tidal action upon the latter has resulted in the periods of 

 rotation and revolution becoming equal to one another. The 

 ancient Arcadians are said to have boasted that their race came 

 into existence before the nioou, but they were probably unaware 

 of the period they claimed for their ancestry (fifty-seven millions 

 of years !). The observations of Schiaparelli having led him 

 to the conclusion that the planet Mercury (which is the nearest 

 known planet to the sun) rotates on its axis in a period equal 

 to that of its revolution round the sun (88 days). Sir George 

 Darwin considers this is due in a similar manner to the tidal 

 action of the central body having lengthened the planet's period 

 of rotation until the latter always presents the same hemisphere 

 towards the sun, just as the moon does towards the earth. 

 The same thing has been asserted of the planet Venus also, 

 but it still seems probable that the shorter period of 23^ hours, 

 determined by the early Italian observers, is the true length 

 of the "day" on the earth's "twin-sister." 



Further modifications, in consequence of increased knowledge 

 of actual existing nebulae and the applications of the principles 

 of energy and thermodynamics, have been proposed from time 

 to time ; and most supporters of the nebular hypothesis no 

 longer believe that in its original condition the "nebula" was 

 even at so high a temperature as that of the sun at present. 

 It is considered probable that the original nebula was largely 

 composed of meteorites, which by collisions during their gradual 

 drawing together would grow hotter and hotter. After a time 

 the central mass would become an intensely hot " white" star. 

 Later on, the loss of heat from radiation exceeding the gain 

 from contraction and condensation, the star would cool down 

 and perhaps finally become a dark body like the companion to 

 Algol. The planets, being smaller than the star or sun round 

 which they revolved, would cool down at a much quicker rate, 

 losing more heat from their surfaces and becoming non- 



