THE MOTION OF THE EARTH. 11 



material universe and its phenomena as dependent upon 

 " active principles" for instance, the cause of gravity 

 whereby the planets and comets preserve their motions 

 in their orbits, and all bodies acquire a degree of motion 

 in falling ; and the cause of fomentation whereby the 

 heart and blood of animals preserve a perpetual warmth 

 and motion the inner parts of the earth are kept con- 

 stantly warmed many bodies bum and shine and the 

 sun himself burns and shines, and with his light warms 

 and cheers all things." 



The earth turns on its axis at the rate of more than 

 1,000 miles an hour, and passes around the sun with 

 the speed of upwards of 68,000 miles in the same time.* 

 The earth and the other planets of our system move in 

 ellipses around a common centre : therefore their motion 

 cannot have been originally communicated merely by 

 the impressed force of projection. Two forces, at least, 

 must have operated, one making the planets tend 

 directly to the centre, and the other impelling them to 

 fly off at a tangent to the curve described. Here we 

 have a system of spheres, held by some power to a great 

 central mass, around which they revolve with a fearful 

 velocity. Nor is this all; the Solar System itself, 

 bound by the same mystic chain to an undiscovered 

 centre, moves towards a point in space at the rate of 

 33,550,000 geographical miles, whilst our earth per- 

 forms one revolution around the sun.f 



* " The revolution of the earth is performed in a natural day, 

 or, more strictly speaking, once in 23h. 56' 4", and as its mean 

 circumference is 24,871 miles, it follows that any point in its equa- 

 torial surface has a rotatory motion of more than 1,000 miles per 

 hour. This velocity must gradually diminish to nothing at either 

 pole. Whilst the earth is thus revolving on its axis, it has a pro- 

 gressive motion in its orhit. If we take the length of the earth's 

 orbit at 630,000,000, its motion through space must exceed 68,490 

 miles in the hour." Enc. Brit. art. Physical Geography. 



\ "Here then we have the splendid result of the united studies 

 of MM. Argelander, 0. Struve, and Peters, grounded on ohserva- 



