THE SIDEREAL UNIVERSE. 745 



Our planet is entirely enveloped by a thick layer of air, 

 which forms round it the softest cushion imaginable. 

 Notwithstanding its apparent lightness, this atmosphere 

 weighs heavily upon all bodies on the earth, and exerts 

 greater pressure in proportion as they offer a larger sur- 

 face. Physiologists consider that each of us has a weight 

 of abont 35,300 lbs. to support, but this great weight 

 is not usually felt, because it is counterbalanced by a 

 counter action equal in all directions, so that the one 

 destroys the other. 



The earth is not rich in respect to satellites, possessing 

 as it does only one, which however is of dimensions ample 



327. Compaiative Dimensions of the Eartli and ]Moon. 



enough as compared to it: this is the moon, the foithful 

 companion of its course. Other planets, it is true, like 

 Jupiter and Saturn, are more richly endowed, and have 

 from four to eight satellites; but again there are others 

 which do not possess any, as is the case with Venus and 

 Mercury. 



