IS TEE M002T DEAD? 



2S1 



etcr, and the entire mass of the moon exceeds 

 the mass of the outermost layer (to a depth of 

 100 miles) in about the proportion of four to one. 

 So that, even on the assumption that all the ex- 

 ternal parts of the moon, to the depth of 100 

 miles, contracted in such a way as to leave cav- 

 ernous spaces in the manner conceived by Frank- 

 land, there would not be nearly enough space 

 for the lunar oceans, supposing them to bear the 

 same proportion to the moon's mass which our 

 ocean bears to the mass of the earth. 



But, though cavernous spaces would not form 

 throughout the interior of a planet, room would 

 yet be found, even to the degree conceived by 

 Frankland, for the waters of the planet. The 

 greatest possible pressure to which the most 

 solid rock can be exposed would not fill the cap- 

 illary spaces which exist throughout the material 

 of the rock, while the pressure on the water at 

 great depths would force it into even minuter 

 than capillary spaces. This has been conclusively 

 shown during experiments entered upon for an- 

 other purpose — viz., to determine the compres- 

 sibility of water. For when, in 1661, Florentine 

 academicians tried to compress water which had 

 been inclosed within a globular shell of gold, 

 they found that the water under great pressure 

 forced its way through the pores of the gold, and 

 stood on the outside of the globe like dew ; and 

 since that time the experiment has been repeated 

 with globes of other metals, a similar result being 

 obtained. 



It follows from these considerations that, as 

 a planet cools, more and more space is formed 

 for the retreat of the planet's seas ; and that in 

 all probability in the extreme old age of a planet, 

 when its whole frame to the very centre has been 

 sufficiently cooled, space enough is thus formed 

 to hold all (he water which had once adorned the 

 planet's surface. 



If we consider the whole history of the moon's 

 cooling, partly as indicated by her actual aspect, 

 partly by the evidence given by the aspect of 

 other planets, and partly as justly inferable from 

 the laws of physics, we shall find abundant rea- 

 son for believing that her seas at any rate might 

 thus have been withdrawn. During the earlier 

 stages of a planet's history, considered in the 

 essay entitled " When the Seas were young " 

 (Cornhill for August and October last), the seas 

 are floating in the form of cloud and vapor above 

 the planet's surface. In the next stage, when 

 the crust is still hot, but not too hot for the 

 waters to vest upon it, the process of cooling 

 must take place more rapidly in the crust of the 

 plauet than in the planet's interior. All this 



time, then, the crust would be contracting upon 

 the nucleus — a process which would leave no 

 cavernous spaces between the crust and the nu- 

 cleus for the waters to retreat to. From time to 

 time the contracting crust would give way, ex- 

 actly as a non-contracting crust would give way 

 under the pressure of an expanding nucleus. The 

 scene of such a catastrophe would be marked 

 thereafter by a great crater at the place where 

 the crust first gave way, and a series of radiating 

 streaks marking the places where the crust was 

 split open all around that spot. The signs of 

 events such as these in the moon's earlier history 

 are very manifest. There is the great lunar cra- 

 ter Tycho, which is clearly visible to the naked 

 eye, near the lower part of the disk of the moon ; 

 and from this as a centre radiations extend in 

 all directions, some of which run right across the 

 visible lunar hemisphere, and probably extend 

 right round the moon. These also can be seen 

 with the naked eye ; and they are so well marked 

 in photographs of the moon that some supposed 

 the earlier photographs by Diaper and Kuther- 

 furd in America, and by De la Rue in this country, 

 were in reality only photographs of a peeled 

 orange, the crater Tycho representing one end of 

 the core, and the radiations corresponding to 

 divisions between the sections of the orange. 

 Besides this most remarkable case, there are six 

 others, centres of radiating streaks on the moon's 

 visible hemisphere, and doubtless others upon 

 the unseen hemisphere. "We have here clear 

 evidence of the tremendous nature of the forces 

 which were at work throughout the moon's frame 

 in the earlier stages of her history, the disturb- 

 ance in particular by which the radiations from 

 Tycho were made having apparently wracked the 

 whole frame of the moon. Directly, indeed, 

 these considerations do not affect the theory we 

 are considering, because no large portion of the 

 lunar seas can by any possibility have retreated 

 beneath the surface during this stage of her ex- 

 istence. But as showing the enormous store of 

 heat which existed at that time (by far the 

 larger part of which must have remained unex- 

 hausted when the next stage began) the consid- 

 eration of these amazing evidences of disturb- 

 ance has an important though indirect bearing 

 on our subject. 



After the crust had parted with the greater 

 portion of the heat which it had possessed when 

 first formed, it would cool, and therefore would 

 contract but slowly. The nucleus, on the other 

 hand, which had before contracted more slowly 

 than the crust, would now contract more rapidly, 

 leaving spaces between itself and the crust. And 



