4 EXPERIMENTS ON 



effect is extremely inconsiderable, when compared 

 with that caused by the bulk of water contained in 

 dissolution in the atmosphere : we may, therefore, 

 in the present inquiry, consider them merely as 

 agents, composing and decomposing perpetually the 

 air; and neglect the consideration of that immediate 

 effect which Smith and Euler have ascribed to 

 them. Should we then succeed, in ascertaining, with 

 any degree of accuracy, the relative degree of mois- 

 ture and dr3'ncss, at the different times of observa- 

 tion, we shall (without neglecting other consi<lera- 

 tions) lay more stress on these results, than on what 

 might be deduced separately from observations of the 

 barometer and thermometer. 



Before entering into the subject of experiments, it 

 will be proper to preface a few words on the motives 

 which induced me to attend particularly to the ef- 

 fects of terrestrial refraction, at the time that I did ; 

 as it will afford an opportunity of giving an account 

 of the data on which I chiefly proceeded. 



Havins: received directions from Major Lamb- 

 TON *, to measure a line near Bangalore, to serve as 

 a base of verification to the trigonometrical operations 

 which were then carried on under his superintend- 

 ance, it occurred to me, when this service was com- 

 pleted, that so favourable an opportunity was not 

 likely to recur, for entering minutely into the sub- 

 ject; for every possible means had been taken, to in- 

 sure as accurate a measurement as could be effected : 

 ;ind this line, together with the elevation of one of 



" refractive power of the substance itself; which probably increases 

 " l)y hcut, and diniinis'ies by cold." 



But this applies chiefly to hard media, such as glass lenses, &c. &c. 

 and may be deemed (for the present) too inconsiderable to require 

 particular notice, where air is the medium. 



* Whoso assistant I then was. 



