THE MOON'S FACE 55 



estimate to about three miles and a quarter. Herschel 

 attacked the problem, armed with a telescope of six 

 feet eight inches focal length, which he speaks of as 

 " a very excellent instrument, equal to any that was 

 ever made." He brought to it also the same " uncom- 

 mon diligence and attention," which made up in some 

 measure for the imperfect instruments of previous 

 astronomers; and he had confidence in himself, in 

 his eyesight, and in the goodness of the work he had 

 done. 



He was struck by the " deep shadows " cast by 

 mountains on the moon's surface. Probably these 

 shadows were then a puzzle to him. But he made 

 one sagacious observation, which subsequent observers 

 have developed into a view of the moon's face alto- 

 gether different from what he started with. On Mons 

 Lacer he writes : " I am almost certain there are two 

 very considerable cavities or places where the ground 

 descends below the level of the convexity, just before 

 these mountains." The moon's face is now known to 

 be pitted with hollows of great extent and depth. 

 Herschel's predecessors called them seas and oceans, 

 of which there are none on the moon. The hills and 

 mountains that rise from these vast cavities do not at 

 the utmost greatly exceed the estimate come to by 

 Herschel, a mile and a half, or a mile and three- 

 quarters in height. But if the height be reckoned 

 above the hollow from which they rise, it may be 

 nearer three times as much. We count the heights of 

 mountains on the earth from the level of the sea. If 

 we reckoned from the bottom of the ocean, our moun- 

 tains will be found considerably to exceed in height 

 those of the moon. It is now known that these 



