244 Trees, Stars, and Birds 



seen extending from it toward the equator. These 

 lines were thought by the astronomer Percival Lowell 

 to be strips of green vegetation bordering streams of 

 water so straight that they must be canals constructed 

 by human beings. 



On the other hand most astronomers, even when 

 provided with very large telescopes, are unable to see 

 anything that convinces them that Mars is inhabited. 

 It is so far from the sun that unless it has a supply of 

 heat of its own, snow would probably not melt in the 

 spring. Possibly the whiteness may be due to frozen 

 carbon dioxid or some other substance which melts at 

 a lower temperature than snow. 



THE MINOR PLANETS 



Early in the nineteenth century four minor planets, 

 or asteroids, were discovered, and nearly 900 are now 

 known. The ones first discovered are probably between 

 100 and 500 miles in diameter ; those recently discovered 

 are only 10 or 20 miles in diameter. All of them put 

 together would make a planet only a small fraction of 

 the size of the earth. 



Like the large planets, the minor planets go around 

 the sun, but in orbits that are more eccentric. That 

 is, their paths are not so much like circles. One minor 

 planet, Eros, revolves about the sun in an orbit which 

 at one point is only 13,500,000 miles outside the earth's 

 orbit. The paths of almost all the others now known 

 lie between the orbits of Jupiter and Mars. ,^. 



There are probably many hundreds of minor planets 

 which have not yet been discovered. Of late years, 



