*&73'] Condition of the Moon's Surface. 35 



should look for the existence of a zone of small bodies like 

 the asteroids. I touch on these points to show the kind of 

 evidence (elsewhere given at length) on which I have based 

 my opinion that the solar system had its birth, and long 

 maintained its fires, under the impact and collisions of bodies 

 gathered in from outer space. 



According to this view, the moon, formed at a compara- 

 tively distant epoch in the history of the solar system, 

 would have not merely had its heat originally generated for 

 the most part by meteoric impact, but while still plastic 

 would have been exposed to meteoric downfalls, compared 

 with which all that we know, in the present day, of meteor- 

 showers, aerolitic masses, and so on, must be regarded as 

 altogether insignificant. It would be to such downfall 

 mainly that the maintenance of the moon's heat would at 

 that time be due, though, as we shall presently see, pro- 

 cesses of contraction must have not only supplemented this 

 source of heat-supply, but must have continued to maintain 

 the moon's heat long after the meteoric source of heat had 

 become comparatively ineffective. 



Now, I would notice in passing that here we may find an 

 explanation of the agreement between the moon's rotation 

 period and her period of revolution. It is clear that under 

 the continuous downfall of meteoric matter in that distant 

 era, the moon must have been in a process of actual growth. 

 She is indeed growing now from the same cause ; and so 

 is the earth : but such growth must be regarded as in- 

 finitesimally small. In the earlier periods of the moon's 

 history, on the contrary, the moon's growth must have pro- 

 gressed at a comparatively rapid rate. Now this influx of 

 matter must have resulted in a gradual reduction of the 

 moon's rate of rotation, if (as we must suppose) the moon 

 gathered matter merely by chance collisions. In the case 

 of a globe gathering in matter by its own attractive power 

 as the sun does, for instance, the arriving matter may (owing 

 to the manner in which the process is effected) serve to 

 maintain and even to increase the rate of rotation ; but in the 

 case of a subordinate body like the moon we must suppose 

 that all effects acting on the rotation would be about equally 

 balanced, and that the sole really effective result would be 

 the increase of the moon's bulk, and the consequent diminu- 

 tion of her rotation rate. Now, if this process continued 

 until the rotation rate had nearly reached its present value, 

 the earth's attraction would suffice not merely to bring the 

 rate of rotation precisely to its present value, but to prevent 

 its changing (by the continuance of the process) to a smaller 



