HYGROLOCY, AND ITS CONNECTION WITH METEOROLOGY." 2^3 



in the Phil. Transactions for 1790 and 179I j the subjects of 

 which I shall here shortly explain, for those of your readers who 

 do not possess the Phil. Trans. 



5. There is no physical instrument, the name of which The hygrome- 

 terminates in meter, as used for measuring the intensity of the measum- o" 

 cause acting upon it, so deserving that name, as the hygrometer moisture in the 

 described in these papers ; for this instrument alone has the ^"'* 

 property of measuring the whole extent of the cause which 

 influences it ; which extent is comprised between two natural and 



opposite extreme points, one of which I shall first describe : it is 

 extreme dryness, or absence of all moisture ; which, there- 

 fore, is an absolute 0. I have proved, in the above papers to p^j^^ ^j. ^^^ 

 the Royal Society, that this point is effectually obtained, by treme drynsss. 

 placing the hygrometer in a close vessel, filled previously with 

 a sufficient quantity of fresh calcined lime, taken red-hot from 

 the kiln. 



6. The principle which led me to this method is, that, evapo- Principle on 



ration being produced by heat, if red-heat is not destructive of ^^"^!* the as- 



. ' ■' , . certamtngit 



a hygroscopic body, it must occasion the evaporation of all the is foun(Jed. 



uncomlined water the latter contains in its pores. And by pre- ,' 



vious experiments on various bodies of that kind, I found, that 



lime, passing from red-heat to extreme moisture, increased in 



proportion of nearly half its weight. I fixed therefore, upon lime, 



and I employed a large vessel, which I filled with red-hot lime. 



When it was cool, that vessel having at the top small openings 



for introducing the hygrometers, (after which they were closed, 



and opened only for taking them out,) I took thus the point 



on a great number of various sorts of hygrometers, of which I 



shall speak hereafter. I have described this vessel in the 



Phil, Transactions 3 it is cylindrical, 1 foot diameter, and 



3 feet high ; I have it still, and when I place in it one of the 



hvsrometers, Uhe of which had been fixed in it 10 years ago, Not varied by 



JO ' ■■ ' time 



I do not find any sensible difference in this point. Thus, 



therefore, the point of extreme dryness is perfectly ascer- 

 tained. 



7. As to the opposite point, that of extreme moisture, I have point of ex, 

 proved in the same paper, that it was surely obtained by trememoi.- 

 immersing the hygrometer into water; where it soon attains a 



point, beyond which it does not go, whatever length of time it 



