xxxviii President'' s Address. [-^ug. 29, 



of Planetary Evolution. By the most refined mathematical analysis 

 he calculates the effect produced on the motion of two rotating viscous 

 bodies by the " bodily " tides each would produce in the other. The 

 primary effect of a tide raised on such a body is to diminish its velocity 

 of rotation ; the tide acts as a brake since the interior portions are 

 revolving as it were in a collar, and gradually the body thus acted 

 on tends to turn always the same face to the other. In the case of 

 the moon this has already come to pass. And further the tidal wave 

 produced on the earth by the moon reacts on the moon and pulls her 

 forward ; now a forward pull of this kind means an enlargement of 

 the moon's orbit so that she is gradually receding from the earth. 

 Following this line of reasoning we see that in ancient times the moon 

 must have been very much closer to the earth than at present, and 

 indeed Prof. Darwin traces her history back to a time when she was 

 revolving close to the earth in a period of between two and three hours. 

 In 1881 he published an investigation, following the same principles, 

 into the early history of the whole solar system ; and he found that 

 the earth and moon formed a singular exception to the other members 

 of the system, owing chiefly to the moon being much larger relatively 

 to its primary than the other satellites and having in consequence a 

 very much greater tide-producing power. He was led to the con- 

 clusion that the satellites of the other planets had never revolved in 

 much smaller orbits than they do at present. Solar tidal friction 

 however played an important part in determining whether a planet- 

 was to have moons or not, if we accept the view that satellites were 

 produced by the disruption of a too-rapidly rotating primary, as by 

 checking the velocity of rotation it checked the tendency to disruption. . 

 Hence we see that Mercury and Yenus, where the solar influence is 

 strong, were unable to rotate with sufficient speed to produce satellites ■ 

 and the earth narrowly escaped the same fate ; while the outer planets,, 

 where the influence of the brake is weak, are well provided with moons.. 



COMETS. 



No great stride has been made in our knowledge of these bodies- 

 in spite of the large number of brilliant ones we have been favoured 

 with, but several advances of considerable importance have been 

 secured. There seems to be little doubt that the heads of comets ^ 

 are aggregations of small particles, the interstices being filled with 

 gaseous vapours, and that the tails are due to electrical repulsion- 



