230 THE BOEDERLAND OF SCIENCE. 



leaving, of course, ample room for every drop of water 

 to flow to that unseen half. The air would also flow 

 to that side. It is not, however, altogether so clear 

 that the air would be concealed in the same way that 

 the water would be. The fact is, one half of the moon 

 is not wholly hidden from our view. There is a 

 * balancing motion ' (technically called the ' libration' ) 

 of the moon, by which she now tilts one part of the 

 farther hemisphere towards the earth, and then another 

 part, with a singular alternation which brings the 

 balancing round so as to affect in turn every part of the 

 moon's edge. And owing to this peculiarity, instead 

 of one half of the moon remaining concealed from us, 

 about forty-two parts out of 100 only are altogether 

 and at all times unseen. It is difficult to believe that 

 an atmosphere coerced so much less than our own 

 (since the moon's attractive power at her surface is but 

 one-sixth of the earth's at her's) would confine itself 

 strictly within limits so narrow. 



But in reality, evidence has been obtained in favour 

 of Hansen's fundamental theory, which, if admitted, 

 disposes altogether of the conclusions based upon that 

 theory. The continental astronomer, Grussew, of Wilna> 

 has very carefully examined some of De la Kue's lunar 

 photographs, taken when the moon was at opposite 

 stages of her balancing motion, and by noting how 

 much the several craters, &c., are displaced, he has 

 found the means of determining the shape of the 

 moon's surface. According to his measurements the 

 greater part of the visible surface of the moon must be 



