On Refraction, 40 



by some seconds. In his paper on the subject, which is 

 divided into four parts, he proves, first, that the mean re- 

 fractions are very nearly the same ibr the same apparent al- 

 titudes throughout the whole extent of the temperate zone; 

 since those which were observed at Paris did not exceed 

 those observed at the Cape of Good Hope but Vp-at most. 

 In the second he determines the absolute quantity of the 

 mean refraction for the apparent height of the pole at Paris, 

 and gives the result of his observations with regard to the 

 latitude of Paris and of the Cape of Good Hope. In the 

 third he gives his table of mean refraction, and another of 

 corrections depending upon the state of the barometer and 

 thermometer; concluding with some reflections on its con- 

 struction and use. In the fourth he compares his new table 

 with the most celebrated of those that had before that time 

 been in use among astronomers ; and he then shows how it 

 agrees with the observations of Bradley, Zanotti and Mayer. 



But by La Caille's Memoir * it appears, that previous to 

 this time M.Mayer had formed and communicated to him a 

 table of astronomic refractions which he computed by means 

 of an algebraic formulaf, the coefficients of which he de- 

 duced from his own observations, and took into account the 

 variations relative to thoseof the barometer and thermometer. 

 He found the alteration of refraction for a depression of 1 5 

 lines in the barometer, the same as for a rise of 10 degrees 

 in the thermometer, and the fariation for each degree of the 

 latter, according to his table, -^ of the whole mean refrac- 

 tion, which he adapted for 28 inches of the barometer, and 

 0° of the thermometer^. This proportion takes place down 

 to 80° of zenith distance. Mayer considered also that the 

 mean refraction is the same for all parts of the earth ; and 

 that the only variation which takes place, depends on the 

 changes of the § weight and temperature of the atmosphere. 



La Caille,in comparing Mayer's Table with observations, 



* Mem. 1755, p. 555. + Vid. Mayers Tables, 1770. 



$ French measure and Reaumur's therm. 



$ It was perceived that the refraction near the horizon at Paris is sensibl/ 

 affected by vapours, and the smoke which arisen from the city, situated north 

 of the observatory. Exhalations and the moisture of the atmosphere have 

 certainly a considerable influence on it, and so has the situation of the place, 

 being more or less elevated. The neighbourhood of a city, mountains or 

 hills, forests, rivers, or marshy plains likewise affect it much ; and La Caille 

 was persuaded that an astronomer never had refractions purely celestial near 

 the horizon, that is, of the nature of those 20° above it ; local circumstances 

 producing such considerable differences in them that he did. not choost 

 to insert in his table those for altitudes below 6°. Cassini de Thury be- 

 lieved that the refraction and its inequality were greater at Paris under similar 

 altitudes, on the south side than on the north, and at 4° he found it 20" more. 

 Mncyci. Meth. 



found 



