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



The discrepancy between the two columns, in the case of 

 refraction, is I own very serious ; but I cannot change my own 

 figures, or see exactly how Le Gentil got his own. The normal 

 refraction according to Bessel for Z. D. 89° SO' is about 29' 3", 

 with a probable error, from experiment, of + 20": Ivory making it 

 28' 31". The former gives nothing below this, but at 90° Z. D. 

 according to Ivory, with the Bar. at 30°, as I have assumed 

 throughout my calculations, and the Therm, at 60 F. in winter and 

 70 F. in summer, which seems reasonable, we get 33' 50" and 

 33' 10" respectively, Bessel's system probably giving in each case 

 about 40" more. My own mean results being 33' 19" and 32' 7": 

 it would seem that the allowance for temperature made by Ivory 

 is hardly sufficient, as Le Gentil says (p. 342) that the difference 

 between the summer and winter temperature at Pondicherry was 

 not more than 5° to 6° (R.) in winter in the morning and 13° to 14° 

 in the afternoon. Any how I do not think the refraction can have 

 been as small as Le Gentil makes it in his summary (p. 248) : it is 

 to be regretted that he kept so much in his mind the phenomenon 

 said to have been seen by the Dutch in Nova Zembla : which can- 

 not in strictness be thought more than interesting. 



I shall conclude this paper with a brief resume of the observa- 

 tions on horizontal refraction made by M. Bouguer in Peru ; which 

 are given and discussed by him in two articles in the Memoires of 

 1739 and 1749. It should be noticed that he only records his 

 actual results, and not his complete observations, or his methods of 

 procedure. 



Experiments in Peru. 



M. Bouguer (9 — 23 April, 1786) found, as he asserts, the refrac- 

 tion for the setting Sun, on the sea-shore, in 1° 1' S. Lat. to be 25' 

 to 29'... at 1° alt. 20|'... The modern tables giving, as the normal 

 refraction, 33' and 24£' about, in these cases, respectively. He 

 thus agrees more or less nearly with Le Gentil in his estimate of 

 refraction generally. 



On the sea-coast, on another occasion, at an alt. of 1°, but at an 

 elevation of 40 toises (= 256 ft.), he found it 22' 15". At Quito, 

 at an altitude of 1400 toises '(= 8960 Eng. ft), he says it was 

 for 2° 20" alt. 12' 1" (the normal beiug 17'), at 3° alt. 9' 33" (norm. 

 14' 36"), at 4° alt. 8' (norm. 11' 51"). 



On the height of Pichincha, near Quito (elevation 527 toises, 

 = 3373 Eng. ft.), he was unsuccessful in obtaining any observations 

 of value, for various reasons : but on Chimborazo, at an elevation 

 of 2388 toises (= 15,283 Eng. ft.) he obtained the following 

 results from the Sun : 



