^0 QUANTITY OF HORIZOSTAL REFRACTION. 



when the telefcope was held vertically, it gave a horizontal 

 view at any level that was found moft eligible. When the 

 water has been calm, I have obferved that the greateft refrac- 

 tion was vifible within an inch or two of its furface, and I have 

 then feen a refradion of fix or feven minutes in the fpace of 

 500 or 400 yards : at other times, I have found it greateft at 

 the height of a foot or two ; but, in this cafe, a far more ex- 

 tenfive view becomes neceflary. 



The firft meafures that I took were on the 23d of September, 

 1800. The water was 2|° warmer than the air, and I found 

 a refra6tion of about 4'. 



Oct. 17. The difference of temperature was 3**, and the 

 refraction 3'. 



Oct. 22. The water was 1)-° warmer than the air, yet the 

 quantity of refra6lIon did not exceed 3'. 



The fmallnefs of the quantity of refra6lion upon this occa- 

 fion, I attributed to the drynefs of the atmofphere, conje£luring 

 that a rapid evaporation might in great raeafure counteradl 

 that warmth which the water would otherwife have commu- 

 nicated to the air. 



From that time, therefore, I have noted not only the heights 

 of the thermometer in the water and in the air, but have added 

 alfo the degrees of cold produced by keeping the bulb of it 

 moiftened for a fufificient time to render it ftationary. In con- 

 firmation of my conje6lure refpei^ing the drynefs of 06t. 22, 

 I have alfo, in the following Table, which comprifes the whole 

 of my obfervations, inferted a column from the Regifter kept 

 at the apartments of the Royal Society, containing the heights 

 of the hygrometer, on thofe mornings when my obfervations 

 were made. 



TABLE. 



