* 296, 2 9 7] Satellites of Jupiter : the Moon: Mars 383 



of our nearest neighbour, the moon. The visible half has 

 been elaborately mapped, and the heights of the chief 

 mountain ranges measured by means of their shadows. 

 Modern knowledge has done much to dispel the view, held 

 by the earlier telescopists and shared to some extent even 

 by Herschel, that the moon closely resembles the earth and 

 is suitable for inhabitants like ourselves. The dark spaces 

 which were once taken to be seas and still bear that name 

 nre evidently covered with dry rock ; and the craters with 

 which the moon is covered are all with one or two doubt- 

 ful exceptions extinct ; the long dark lines known as 

 rills and formerly taken for river-beds have clearly no 

 water in them. The question of a lunar atmosphere is 

 more difficult : if there is air its density must be very small, 

 some hundredfold less than that of our atmosphere at the 

 surface of the earth ; but with this restriction there seems 

 to be no bar to the existence of a lunar atmosphere of 

 considerable extent, and it is difficult to explain certain 

 observations without assuming the existence of some atmo- 

 sphere. 



297. Mars, being the nearest of the superior planets, is 

 the most favourably situated for observation. The chief 

 markings on its surface provisionally interpreted as being 

 land and water are fairly permanent and therefore 

 recognisable; several tolerably consistent maps of the 

 surface have been constructed ; and by observation of 

 certain striking features the rotation period has been 

 determined to a fraction of a second. Signer Schiaparelli 

 of Milan detected at the opposition of 1877 a number of 

 intersecting dark lines generally known as canals, and as 

 the result of observations made during the opposition of 

 1881-82 announced that certain of them appeared doubled, 

 two nearly parallel lines being then seen instead of one. 

 These remarkable observations have been to a great extent 

 confirmed by other observers, but remain unexplained. 



The visible surfaces of Jupiter and Saturn appear to be 

 layers of clouds ; the low density of each planet (1*3 and 

 7 respectively, that of water being i and of the earth 5*5), 

 the rapid changes on the surface, and other facts indicate 

 that these planets are to a great extent in a fluid condition, 

 and have a high temperature at a very moderate distance 



