'TERIIESTRIAL REFRACTION. 7 



Taking it for granted (as perhaps the present ex- 

 periments will he deemed to allow) that where the 

 moisture of the atmosphere is greatest, the refraction 

 at that moment is also greatest (and vice versa) 

 wherever, in the annexed tables, the observation 

 rigorously agrees with this theor}', the mark (§) is 

 placed underneath*; where the coincidence of mois- 

 ture and refraction is not absolutely exact, the ap- 

 proximation is pointed out by a cross I %, shewing, 

 by the number of its bars, how near it comes to 

 perfect consistency. 



With a view to draw the attention still closer to 

 the effects of moisture on refraction, two columns 

 of differences, which were kept at the time of obser- 

 vation, have been preserved. The column which 

 relates to the angles being marked +^, according as 

 these have been measured greater or less, at the two 

 nearest observations ; and that of the contempora- 

 neous relative moisture being marked M, or D, ac- 

 cording as the atmosphere had changed from a 

 moister to a drier state, and vice versa. 



As this coincidence has been very general, I found 



* It is to be noticed, however, that nioriiing and evening obser- 

 Tations are compared separately, neglecting those taken in the 

 middle of the day; for these, from the great motion which then 

 disturbs the atmosphere, must necessarily be very imperfect. The 

 only reason why these observations are not left out altogetlier, is, 

 that however imperfect they may be, as to a second or two, they 

 contribute nevertheless to establish the proof of the relation of mois- 

 ture to refraction ; for it is to be observed, that (when the weather is 

 perfectly clear of rainy clouds) the refraction is never so inconsider- 

 able as in the heat of noon, when the sun shines brightest, and when 

 the tremor of the atmosphere is most considerable : and the reason 

 6f this seems to be, " because the atmosphere, however disturbed, 

 ** is then generally in its driest state." There was so little refraction 

 at that hour, that the elevations of the Munfapicm frequently bring 

 it out negative; an absurdity which arises both from the impossibi- 

 lity of taking the angles accurately, and the very small refraction 

 theu prevailing. 



Jl 4 



