THE CONSTANT FACE OF THE MOON. 59 



far vaster than any tides which the moon can create on the 

 earth. But it may be said that as the moon contains no 

 water it seems idle to talk of the tides that might have 

 been produced in oceans if they had existed. It is no 

 doubt true that the moon contains no visible liquid water 

 on its surface at the present time ; it is, however, by no 

 means certain that our satellite was always void of water ; 

 it is not at all impossible that spreading oceans may have 

 once occupied a large part of that surface now an arid 

 wilderness. The waters from those oceans have vanished, 

 but the basins they presumably filled are still left as 

 characteristic features on our satellite. For our present 

 argument, however, it is really not material that the moon 

 should ever have had oceans as we understand them. Have 

 we not seen that there was a time when the very mass of 

 the moon itself appears to have been largely, if not wholly, 

 liquefied ? The water at those remote periods must have 

 been suspended in the form of vapour around the more 

 solid parts of the glowing globe. But tides can be 

 manifested in other liquids besides that which forms our 

 seas. In fact if the basins of our great oceans were filled 

 with oil or with mercury, or even with molten iron instead 

 of water, the moon would still cause tides to ebb and flow, 

 no matter what the material might be, so long as it pos- 

 sessed to some extent the properties of a liquid. It need 

 not be a perfect liquid, for any material which is in some 

 degree viscous, like honey or treacle, would still respond 

 to tidal influence, though not, it may be well believed, with 

 the same alacrity and freedom of movement as would a 

 fluid of a more perfect character. In the molten moon 

 itself, throughout the very body of our satellite, the tidal 



