346 , On Refraction. 



De la Grange*, Euler 7 , Gregorv , Hodgson , Huygens r< % 

 K^mp 11 , Lambert 12 , Laplace 1 *, Mayer 14 , INiewton 15 , Oriani 16 , 

 Thomas Simpson 17 , Brook Taylor' 8 , Heinsius K; , Tobias 

 Mayer", La Hire*', d'Alembert". 



It was conjectured by many of the early writers, that the 

 refraction was' subject to variations depending upon the 

 weather: but it then amounted to little more than a con- 

 jecture, on account of the indifferent manner in which 

 astronomic instruments were divided. Picard found by 

 meridian altitudes of the sun in 166*9, that it was greater 

 in winter than in summer. He found also that it was less 

 by day than by night. In the observations given at the 

 end of his journey to Uraniburg*, to settle the latitude of 

 that place, and its difference of longitude from Paris, for 

 the purpose of comparing the observations of Tycho Brahe 

 with those made at the Royal Observatory of Paris, he 

 found the horizontal refraction for the first limb of the sun 

 that made its appearance above the horizon there 33' 2", 

 and for the second 32' 37". So that in the small interval of 

 time that the sun took to rise, the refraction was diminished 

 25 seconds by the warmth arising from the sun's presence. 



A quadrant being also directed by him from the top of 

 Mount Valerian towards the summit of the church of Notre 

 Dame at Paris, he found the depression 20' ; but the sun 

 had scarcely risen, when it was increased to 22 / ; exhala^ 

 tions being raised by the sun's presence, and the medium 

 between Paris and Mount Valerian become more equal ; 

 whereas, before the sun rose, the air of Paris was more dense 

 than that of' Mount Valerian f. 



The density of the atmosphere being the immediate cause 

 of the refraction, it was very natural to suppose that it must 

 decrease as this density became less; whether by causes 

 which diminished its weight, or by the expansion produced 

 by heat : and, indeed, astronomers were not long after this, 



6 NouveauxMemoiresde Berlin, vol. Hi. ? Mem.de Berlin, 1754, torn. x. 



* Astronomy, vol. i. p. 3.58. edit, of 1715, in 8vo. 



9 Mathematics, vol. i. p. 867. Fluxions, p. 133. 



• IO Traite de la Lumiere, p. 44. Dioptrica, 4to, 1703. 

 K ' >» Analyse des Refract. Astr. et Terres. 4to. Strasburg 1 , 1799. 



10 Les Proprietes Remarquables de la Route de la Lumiere. A la Haye, 

 1759. Another edition in German, 1773. 



'3 Mecanique Cele6te, vol. iv. p. 231. » 



M Tables, 1770. »5 Principia, b. i. sect. 14. 



,6 F,phem. de Milan, 1788. »7 Mathematical Dissertations, 1743. 



18 Methodus Incrementorum, 4to. Lond. 1715. Propos. 27, p. 108. 



>9 DissertatiodeComputo refractionum Astron. 4io. Leipsig, 1749. 



™ De Refractionibus Astronomicis, 4to. Altorf. 1781. 



21 Mem. de l'Acad. pour 170L>, p. 52. 



52 Opuscules Mathematiques, torn. viii. p. 297. 



• Mem. de l'Acad. av.s.ren.. torn. i. f Eucycl. Meth. a,rt. Refraction. 



before 



