26 



Dr J. B. Pearson, On the experiments made by [Nov. 22, 



can here come into account. I have marked with a positive sign 

 (+) the cases where the refraction must have been greater than 

 the normal, and with a negative (— ), when it must have been 

 less. By the normal, I mean that obtained by Ivory's Tables, 

 which seem to me very convenient for very low altitudes, and, as I 

 have mentioned already, at an altitude of half a degree only vary 

 from those of Bessel by 33 seconds. 



Next I will give the error, as I may call it, of the Sun's altitude 

 at the moment of its emerging from behind the horizon, false or 

 true, in summer or in winter : that is the amount by which the 

 measured Zenith distance differs from that suggested by theory, 

 a discrepancy which, as will be seen from Le Gentil's description of 

 his work, must mainly be due to refraction, but which also may 

 have been slightly due to variations in the depression of the visible 

 sea-horizon, though the latter seems always at Pondicherry to have 

 been greater than would have been expected by theory ; the dip 

 given in the books for 40 (Paris) feet, being about 6' 25 ': whereas 

 he says that his instrument always made it about 8' 50". I may 

 observe, in passing, that with a theodolite, I found the same result 

 at Vardo in Finmark ; so far, that the dip given by the instrument 

 was always greater than my elevation required, though from 

 various circumstances I could not determine the latter, about 30 

 feet, with accuracy. 



Winter, 1769. 



Summer, 1769. 



N.B. — As already mentioned, the sign (+) signifies that the 

 refraction is greater than would be expected, (— ) that it is less. 



