288 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



come on the side where they would actually be found, if the drawing- 

 were in reality in relief or the reverse. 



The beautiful facsimile of a lunar photograph, which we owe to 

 the courtesy of M. Warren Delarue, and which forms the eleventh 

 plate of the fourth edition of The Heavens, is well fitted for the 

 experiment of which we speak. The volcanic craters of the lunar 

 mountains appear in one case like the hollow shafts of a cone, which 

 they really are, and, in the other aspect, they resemble inverted 

 bubbles. 



