162 HISTORY OF PHYSICAL ASTRONOMY. 



"With reference to the Maps of Cotidal Lines, mentioned iu the 

 text, I may add, that we are as yet destitute of observations which 

 should supply tta means of drawing such lines on a large scale in the 

 Pacific Ocean. Admiral Liitke has however supplied us with some 

 valuable materials and remarks on this subject in his Notice sur les 

 Marees Ptriodiques dans le grand Ocean Boreal et dans la Mer 

 Glaciale ; aud has drawn them, apparently on sufficient data, in the 

 White Sea.] 



CHAPTER V. 



DISCOVERIES ADDED TO THE NEWTONIAN THEORY. 



Sect. 1. Tables of Astronomical Refraction. 



WE have travelled over an immense field of astronomical aud math- 

 ematical labor in the last few pages, and have yet, at the end of 

 every step, still found ourselves under the jurisdiction of the New- 

 tonian laws. We are reminded of the universal monarchies, where a 

 man could not escape from the empire without quitting the world. 

 We have now to notice some other discoveries, in which this reference 

 to the law of universal gravitation is less immediate and obvious ; I 

 mean the astronomical discoveries respecting Light. 



The general truths to which the establishment of the true laws of 

 Atmospheric Refraction led astronomers, were the law of Deflection of 

 the rays of light, which applies to all refractions, and the real structure 

 and size of the Atmosphere, so far as it became known. The great 

 discoveries of Homer and Bradley, namely, the Velocity of Light, the 

 Aberration of Light, and the Nutation of the earth's axis, gave a new 

 distinctness to the conceptions of the propagation of light in the minds 

 of philosophers, and confirmed the doctrines of Copernicus, Kepler, 

 aud Newton, respecting the motions which belong to the earth. 



The true laws of Atmospheric Refraction were slowly discovered. 

 Tycho attributed the apparent displacement of the heavenly bodies to 

 the low and gross part of the atmosphere only, and hence made it 

 cease at a point half-way to the zenith ; but Kepler rightly extended 

 it to the zenith itself. Dominic Cassini endeavored to discover the 

 law of this correction by observation, and gave his result in the form 



