ASTRONOMY. 239 



globular form may be inferred by analogy ; as all the other heavenly 

 bodies which are visible to us are globes, there is little reason to doub 

 mat the earth is so likewise. 



About two thirds of the earth are covered with water, whose surface is 

 rounded to conform with the general shape of the earth. On this surface 

 we can sail round the earth in all parts of it, and in all directions ; this 

 nas been proved repeatedly by navigators. Our own experience also 

 proves this ; for example, when one leaves the coast in a ship, mountains, 

 euinces, all prominent objects sink by degrees, and at length disappear as 

 if they had sank into the ocean. Now this effect is not to be attributed 

 to distance, which only causes objects to appear smaller, but when we 

 lose sight of land from the deck we perceive it again by ascending the 

 masts. The same is observed when a ship leaves the shore. It declines 

 by little and little, and finally disappears, descending below the horizon 

 like the sun at its setting. These phenomena, which are observed 

 continually, and in all directions, prove that the surface of the sea is 

 convex, and that it is by its interposition that distant objects are 

 concealed. 



The motion of the earth in her orbit round the sun, is called her 

 annual motion, and that round her own axis, her diurnal motion, 

 which at the equator is about one thousand and forty-two miles an hour. 

 The earth is surrounded by a compound fluid substance, called the atmos- 

 phere, which consists of air mingled with aqueous vapours, and other 

 exhalations from her surface. It has been stated before, that the orbit 

 of the earth is not a perfect circle, but inclines to the Elipse, and that 

 the sun is not exactly in its centre. From this cause the earth is some 

 days longer in passing one half of her orbit than she is in traversing the 

 other. 



THE MOON 



Is the satellite of the earth, and while she revoives round, her primary 

 revolves also round her common centre, the sun ; and by her thus occupy- 

 ing the earth in her course round the sun, she is called a secondary 

 planet, or satellite. The diameter of the moon is only two thousand 

 one hundred and seventy miles, and her distance from the earth is two 

 hundred and forty thousand miles. Her period of revolution round the 

 sun is, of course, the same as that of the earth. That the moon is an 

 opaque body, shining only with reflected light, is evident from the 

 different appearances which she assumes, for if she shone by her own 



