1880.] 



Blot and others on horizontal refraction. 



25 



and 70°. F. in summer, Le Gentil merely saying that it did not 

 vary much at the two seasons. It should be noticed that he gives the 

 apparent time with great accuracy, as ascertained by the method of 

 Equal Altitudes; and I have found that these Time observations 

 verify themselves to considerable nicety, so that the times he gives 

 may be taken as known with comparative certainty, assuming the 

 Long, and Lat. of Pondicherry, as given now in the Connais- 

 sance des Temps, viz. 11° 55' 40" N. Lat. 5h. 9m. 57s. E. Long, of 

 Paris. 



Working on this method, I obtain the following table of com- 

 parison between the true or reduced Zenith distance of the Sun's 

 centre, as obtained by the observations, and that given by the Sun's 

 theoretical place at the same moment : 



Winter, 1769. 



Summer, 1769. 



In all these observations there can be no change ascribed to 

 the apparent sea-horizon, because the point of observation was 

 the moment of the Sun's passing the wire of the telescope, which 

 pointed to a known Zenith distance. Refraction alone therefore 



