THE SKY. 



greater angle with the line of vision. For a short time 

 before and after the equinoxes of Saturn, the rings become 

 invisible, owing to the earth and sun being on opposite 

 sides of them, as may be seen in fig 21, so that the darkened 



side is turned towards the earth and the 

 edge, which is the only part illuminated, is 

 towards the sun. 



The inner ring of Saturn is supposed to be 

 composed of watery vapour, as it is somewhat 

 transparent, but the outer ones are solid, 

 which is shown by the shadow they cast upon 

 the planet, and the shadow it casts upon 

 them in different positions (figs. 22 and 23). 

 Besides these rings Saturn has eight satellites 

 or moons, which revolve in a plane nearly 

 parallel to that of the rings and exterior to 

 them. It has been calculated that Saturn 

 weighs only 100 times more than the earth, 

 although it is somewhere about 900 times 

 larger, from which it is concluded that the substance of 

 which Saturn is made must be about one-ninth the density 

 of this earth, half the density of water, or about the same 

 as cork. Saturn is very much flattened at the poles, so 

 much so that the equatorial diameter is a tenth more than 

 the axial diameter, which difference is distinctly visible 

 through good glasses. 



Still further into space, at double the distance of Saturn, 



o 



s 



FIft. 21. 



