sict. iv] DISSERTATION SECOND. 131 



sive according to their situation in respect of the sun. 

 These are the only inequalities of the moon's motion known 

 before the theory of Gravitation, and, except the two first, 

 are all the discoveries of Tycho. 



The atmospherical refraction, by which the heavenly 

 bodies are made to appear more elevated above the hori- 

 zon than they really are, was suspected before the time 

 of this astronomer, but not known with certainty to exist. 

 He first became acquainted with it by finding that the 

 latitude of his observatory, as determined from observa- 

 tions at the solstices, and from observations of the greatest 

 and least altitudes of the circumpolar stars, always differed 

 about four minutes. The effect of refraction he supposed 

 to be 34' at the horizon, and to diminish from thence up- 

 ward, till at 45 p it ceased altogether. This last supposi- 

 tion is erroneous, but at 45° the refraction is less than 1', 

 and probably was not sensible in the altitudes measured 

 with his instruments, or not distinguishable from the errours 

 of observation. An instrument which he contrived on 

 purpose to make the refraction distinctly visible, shows 

 the scale on which his observatory was furnished. It was 

 an equatorial circle of ten feet diameter, turning on an axis 

 parallel to that of the earth. With the sights of this equa- 

 torial he followed the sun on the day of the summer sol- 

 stice, and found, that, as it descended towards the horizon, 

 it rose above the plane of the instrument. At ils setting, 

 the sun was raised above the horizon by more than its own 

 diameter. 



The comet of 1570 was carefully observed by Tycho s 

 and gave rise to a new theory of those bodies. He found 

 the horizontal parallax to be 20', so that the comet was 

 nearly three times as far off as the moon. He considered 

 comets, therefore, as bodies placed far beyond the reach 

 pf our almosphere, and moving round the sun. This was 



