36 Condition of the Moon's Surface. [January, 



value. It may be added that the increase in the moon's 

 rate of revolution, as she herself and the earth both grew 

 under meteoric downfall towards their present dimen- 

 sions, would operate in a similar way, — it would tend to 

 bring the moon's rate of revolution and her rate of rotation 

 towards that agreement which at present exists. 



If we attempt to picture the condition of the moon in 

 that era of her history when first the process of downfall 

 became so far reduced in activity as to permit of her cooling 

 down, we shall be tempted, I believe, to consider that 

 some of the more remarkable features of her globe had their 

 origin in that period. It may seem, indeed, at a first view, 

 too wild and fanciful an idea to suggest that the multi- 

 tudinous craters on the moon, and especially the smaller 

 craters revealed in countless numbers when telescopes of 

 high power are employed, have been caused by the plash of 

 meteoric rain, — and I should certainly not care to maintain 

 that as the true theory of their origin ; yet it must be re- 

 membered that no plausible theory has yet been urged 

 respecting this remarkable feature of the moon's surface. 

 It is impossible to recognise a real resemblance between 

 any terrestrial feature and the crateriferous surface of the 

 moon. As blowholes, so many openings cannot at any time 

 have been necessary, whatever opinion we may form as to 

 the condition of the moon's interior and its reaction upon 

 the crust. Moreover, it should be remembered that our 

 leading seismologists regard water as absolutely essential 

 to the production of volcanic disturbance (the only form of 

 disturbance which on our earth leads to the formation of 

 cup-shaped openings). If we consider the explanation ad- 

 vanced by Hooke, that these numerous craters were pro- 

 duced in the same way that small cup-shaped depressions 

 are formed when thick calcareous solutions are boiled and 

 left to cool, we see that it is inadequate to account for lunar 

 craters, the least of which (those to which Mr. Birt has 

 given the name of craterlets) are at least half a mile in 

 diameter. The rings obtained by Hooke were formed by the 

 breaking of surface bubbles or blisters,* and it is impossible 

 for such bubbles to be formed on the scale of the lunar 

 craters. Now so far as the smaller craters are concerned, 

 there is nothing incredible in the supposition that they were 



* " Presently ceasing to boil," he says of alabaster, "the whole surface will 

 appear covered all over with s-mall pits, exactly shaped like those of the moon." 

 " The earthy part of the moon has been undermined," he proceeds, " or 

 heaved up by eruptions of vapour, and thrown into the same kind of figured 

 holes as the powder of alabaster." 



