COMMON THINGS THE EARTH. 



once in twenty-four hours ; 2ndly, from the rotation of the globe 

 of the earth in the same time round that diameter which is in 

 the direction of the axis round which the firmament appears to 

 revolve. 



There is absolutely no other supposition possible but one or 

 other of these. The rejection of either necessarily throws us upon 

 the adoption of the other. 



But it may be required that we should show how the rotation of 

 the earth upon an axis passing through the poles would cause the 

 apparent diurnal rotation of the firmament. 



15. Let us assume that the earth is a globe revolving uniformly 

 on its axis in twenty-four hours. The universe around it is 

 relatively stationary, and the bodies which compose it being at 

 distances which mere vision cannot appreciate, appear as if they 

 were situate on the surface of a vast celestial sphere in the centre 

 of which the earth revolves. This rotation of the earth gives to 

 the sphere the appearance of revolving in the contrary direction, 

 as the progressive motion of a boat on a river gives to the 

 banks an appearance of retrogressive motion; and since the 

 apparent motion of the heavens is from east to west, the real 

 rotation of the earth which produces that appearance must be from 

 west to east. 



How this motion of rotation explains the phenomena of the 

 rising and setting of celestial objects is easily understood. An 

 observer placed at any point upon the surface of the earth is 

 carried round the axis in a circle in twenty-four hours, so that 

 every side of the celestial sphere is in succession exposed to his view. 

 As he is carried upon the side opposite to that in which the sun is 

 placed, he sees the starry heavens visible in the absence of the- 

 splendour of that luminary. As he is turned gradually towards 

 the side where the sun is placed, its light begins to appear in the- 

 firmament, the dawn of morning is manifested, and the globe con- 

 tinuing to turn, he is brought into view of the luminary itself, 

 and all the phenomena of dawn, morning, and sunrise are 

 exhibited. "While he is directed towards the side of the firmament 

 in which the sun is placed, the other bodies of inferior lustre are 

 lost in the splendour of that luminary, and all the phenomena of 

 day are exhibited. When by the continued rotation of the globe 

 the observer begins to be turned away from the direction of the 

 sun, that luminary declines, and at length disappears, producing 

 all the phenomena of evening and sunset. 



Such, in general, are the effects which would attend the motion. 



of a spectator placed upon the earth's surface, and carried round 



with it by its motion of rotation. He is the spectator of a gorgeous 



diorama exhibited on a vast scale, the earth which forms his station 



56 



