On Refraction, 345 



on the subject ; and, perhaps, bring others to light of as 

 great or greater importance*. 



The refraction of the north being so considerable, is very 

 useful to the inhabitants, who are deprived of the sun's 

 light during many months ; as it makes the sun rise much 

 earlier, and set much later to them, than it otherwise would. 



About the year 1725, Mr. Flamsteed, the English Astro- 

 nomer Royal, published his tablet computed from his own 

 observations : and this was the one commonly used in En- 

 gland for many years afterwards. 



Sir Isaac Newton also constructed one J from theory, 

 which was first published by Dr. Halley in the Philosophical 

 Transactions, No. 368, for 1 721 . He made the horizontal re- 

 fraction 33' 45"; whereas Mr. Flamstecd's was only 33' 0". 



But although the refraction might be determined within 

 a few seconds at all altitudes by observation ; yet, the 

 law of its increase from the zenith to the horizon was a 

 subject that occupied the principal mathematicians and 

 astronomers for more than a centurv§. Newton having 

 discovered the general principles of attraction, found that 

 the refraction was a consequence of this law of nature ; and 

 that it arose from the attraction of the atmosphere on the 

 particles of light. On this principle the curve which a ray 

 of light describes might be determined ; since it is succes- 

 sively attracted by different layers of the atmosphere, in- 

 creasing in density as they approach the earth, and, conse- 

 quently, bending the ray more and more from the right line 

 which it described in the vacuum previous to its reaching 

 the atmosphere. There are many authors who have en- 

 deavoured to find from theory the curve described by this 

 ray in its course, by the assumption of various hypotheses : 

 but perfection and our attempts to arrive at it, as is well 

 observed by the elder Cassini in discoursing on this sub- 

 ject, are like the progress of certain curves and their asym- 

 ptotes. The principal of these writers on the subject arc, 

 Bernouilli 1 , Boscovich 2 , Bouguer 3 , Cassini 4 , Des Cartes 5 , 



* Encyelop. Meth. art. Refraction. 



f Hist. Celest. vol. i. p. 396; also Hodgson's Math. vol. i. p. 367. Long'* 

 Astronomy, p. 254. ± Long's Astr. p. 2.54. 



§ In 1714, Cassini published in Mem. de l'Acad. for that year, some me- 

 thods of finding the refraction by observation, and of determining its quan- 

 tity by theory. He has also given a table of it for the first .i0° of altitude, 

 computed, first, according to a rectilinear, and, secondly, according to a cir- 

 cular hypothesis which he there assumes. 



1 Hydrodyn. 1738, p. 221. 3 Oper. torn. ii. 



3 Prix de 1729. Memoires, 1739, p. 407 •, 1749, p. 75. 



4 Epist. ad Montanari, 1665. Refrassioni eParallosse, &c. 1671. Mem. 

 for 17 11, and his Astr. vol. i. p. 11. Paris, 1740, in 2 vols.4to. 



5 Dioptrique, 4to. Paris, 1637. De 



