QUANTITY OF HORIZONTAL REFRACTION, ^l^ 



referrible to difference of temperature alone. After a fuccef- water, which ^ 

 lion of weather fo hot that the thermometer, during one month ^^^^f ^^ ^j^^" "' 

 preceding, had been 12 times above 80°, and on an average of loweftftratumof 

 the month at 68«, the evening of that day (Auguft 22, 1 800) j^f^^^^JT^^jy " 

 was unufually cold, the thermometer being 55'^. The water power, 

 might be fuppofed to retain the temperature it had acquired 

 during a few weeks preceding, and, by warming the ftratum of 

 air immediately contiguous to it, might caufe a diminution of 

 its refradive denfity, fufficient to effeft this inverted curvature 

 of the rays of light, in the manner formerly explained. As I 

 was at that time unprovided with inilruments of any kind, I 

 had it not in my power to eftimate the quantity of refradion, or 

 temperatures; and can only fay that, to my hand, the water 

 felt in an uncommon degree warmer than the air. 



Being thus furnifhed with an unexpected field for obferva- Particular ac- 

 tion, I from that time took fuch opportunities as fimilar changes *^"""'°^.*'** 

 of the weather afforded me, of examining and meafuring the 

 quantities of refraftion that might be difcovered by the fame 

 means over another part of the river, that I found mofl fuited 

 to my convenience. 



The fituation from which the greater part of my obfervations 

 were made, was at the S. E. corner of Somerfet houfe. The 

 view from this fpot extends under Blackfriars bridge, towards 

 London bridge, upwards of a mile in length, and in the oppo- 

 fite direction through Weflniinfler bridge, which is three 

 quarters of a mile diftant. 



Such diftances are however by no means neceffary ; and in- 

 deed the air over the river, in cold weather, is generally, or 

 at leaft very frequently, not fufficiently clear for feeing diflinCtly 

 to fo great diftances. For, fince the winds which are moft 

 Jikely to effed a fufficient change of temperature, on account 

 of their coidnefs, are ufually from the E. orN. E. the principal 

 fmoke of the town is then brought in that diredion, and hovers, 

 like a denfe fog over the courfe of the river. This circum* 

 fiance deprived me of many opportunities which the changes 

 of the thermometer indicated to be favourable for niy purpofe, 

 and obliged me often to make u(e of fhorter diftances than I 

 ftiould olherwife have chofen, by bringing the line of fight as 

 near as I could to the furface of the water. 



For this purpofe, I had a plane refle<5tor fitted to the objed- 



end of a fmall pocket telefcope, at an angle of 45*, fo that. 



Vol. VI. — Septembku. E wbeu 



