4UU 



SELENOTOPOGRAPIiy. 



CHAPTER 

 XXVIII. 



Lunar raapf 



taia or ridge. We know that the shadows of the ohjects 

 around us are longer or shorter just as the sun has 

 attained to a lesser or greater height above the horizon ; 

 and as the relations of that orb to the moon are per- 

 fectly, understood, it is easily discovered what his eleva- 

 tion is above the horizon of any body casting a shadow 

 there ; so that the shadow is an accurate indication of 

 both the form and the magnitude of the mountain from 

 which it is thrown. It is the same with those remark- 

 able caverns. Suppose a deep hole in a table, and let a 

 candle approach it, — the deeper the pit, the nearer must 

 the candle be before the light reach its bottom : and ex- 

 actly thus must the position of the sun, when the pro- 

 fundities of these lunar pits are pierced through, enable 

 us to judge how far they descend into the body of that 

 globe. Acting on these simple principles, and therefore 

 watching the crescent edge through many lunations, 

 several celebrated astronomers have constructed maps of 

 our satellite. Without referring to the older labourers, 

 who wrought under the disadvantage of imperfect tele- 

 scopes, I must distinguish Schroeter, whose enthusiasm 

 in this field was worthy its dazzling object. But it is to 

 Maedler, now of Dorpat, that we owe the first accurate 

 selenotopography. This excellent and industrious ob- 

 server has, in conjunction with M. Baer of Berlin, drawn 

 a map of the moon's surface, of three feet in diameter; 

 of which, I believe, this at least may be said, — it is more 

 accurate than any existing chart of either hemisphere of 

 our own globe." 



It may seem, at first sight, altogether extravagant to 

 speak of more perfect maps existing of the moon than of 

 the earth ; but a very little reflection will show that, 

 while the delineator of the surface of the moon literally 

 draws that which he sees, it is only by a complicated 

 and laborious system of triangulation and measurement, 

 that charts and maps can be constructed, even of a very 

 small portion of the earth's surface. Professor Nichol 

 thus sums up the observations of various observers in 



