ROTATION. 



12. A few hours' attentive contemplation of the firmament at 

 night will enable any common observer to perceive, that although 

 the stars are, relatively to each other, fixed, the hemisphere, as a 

 whok, is in motion. Looking at the zenith, that is the point 

 directly above our head, constellation after constellation will 

 appear to pass across it, having risen in an oblique direction from 

 the horizon at one side, and, after passing the zenith, descending 

 on the other side to the horizon, in a direction similarly oblique. 

 Still more careful and longer continued observation, and a com- 

 parison, so far as can be made by the eye, of the different directions 

 successively assumed by the same object, creates a suspicion, which 

 every additional observation strengthens, that the celestial vault 

 has a motion of slow and uniform rotation round a certain diameter 

 as an axis, carrying with it all the objects visible upon it, without 

 in the least deranging their relative positions or disturbing their 

 arrangement. 



When these loose impressions of the senses are submitted to the 

 more exact means of observation which are at the disposition of 

 astronomers, it is found that all the appearances of the heavens, 

 the rising and the setting of the stars, the sun and the moon, their 

 apparent motion in ascending to, passing, and descending from, 

 their several points of culmination, is that of a sphere revolving 

 with an uniform motion round the diameter which is directed to 

 the pole. 



The world we inhabit therefore would, to judge from these 

 phenomena, seem to be fixed in the centre of a hollow sphere of 

 vast magnitude. On the concave surface of this hollow sphere 

 thus surrounding us at an immeasurable distance all the stars 

 appear to be placed. This sphere, carrying the whole creation, 

 upon it, appears to revolve round our world. It makes a complete 

 'revolution in twenty -four hours.* By this rotation, the diurnal 

 appearances of the rising and setting of all the heavenly bodies are 

 perfectly explained. 



13. The ancients who, as has been stated, affirmed the reality 

 of this motion of the celestial sphere, gave to the whole creation 

 around the earth, the name UNTVEESE ; from two words, Uxus, one, 

 and VEBSTTM , turning or rotation ; because they assumed that by 

 an imaginary force, called the PEmrM MOBILE, or first impulse, 

 this rotatory motion had been imparted to the firmament, which 

 ever afterwards retained it. 



14. It is easy to perceive that the apparent diurnal rotation of 

 the firmament round the earth may arise indifferently from either 

 of two causes : 1st, from such a real rotation of the firmament 



* More exactly 23 h 56 m 4'09 , but for the present the cause of this 

 difference need not be noticed. 



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