40 Proposed Improvement of the Hygrometer. 



that the air is charged with water to the maximum of its 

 temperature — a circumstance which renders the indications 

 of the hygrometer highly useful and necessary. The hy- 

 grometer should express— To what column of mercury the 

 water-gas of the air corresponds ? and at the same time de- 

 termine the absolute quantity of the gas ; and, the tempe- 

 rature of the air being known — How much of this gas it 

 can take up beyond what it already holds, and how soon 

 t,he exhalation thereof can take place? Our usual hygro- 

 meters of hair, and of whalebone, are, in this respect, very 

 imperfect : the results from them are not much to be relied 

 on, having always a relation to the temperature in which 

 the examination is made. v 



Dalton made use of a very plain instrument for his hy- 

 grometrical essays : he filled a long cylindrical glass vessel 

 with cold well water, and when the dew appeared to coat 

 the outside, he decanted the water, and wiped the glass well 

 with linen, after which he returned the water, and this he 

 repeated until the glass ceased to appear moist upon the in- 

 troduction of the water ; when he, by means of the ther- 

 mometer, examined the temperature of the water so pour- 

 ed in : he then found the degree of heat at which the air 

 might prove saturated (if I might so say) with the contained 

 water-gas, and in consulting his tables he learned what co- 

 lumn of mercury coincided therewith, he being already ac- 

 quainted with the temperature of the air. This simple ap- 

 paratus served all his purposes. 



We nevertheless may easily see, that although this me- 

 thod is built upon a true principle, yet it will prove to be 

 both inconvenient, tedious, and defective, as the precise 

 temperature at which the glass should cease to appear moist 

 cannot possibly be attained. Therefore, to obtain a greater 

 certainty in the result, though scarcely with less trouble, 

 I altered Dalton's plan in the following manner : 



Let us suppose that the air which we are about to exa- 

 mine is at 20°, and that a glass of ordinary spring water, 

 generally at 7° when recently taken from the spring, is be- 

 dewed in this air. The difference between the temperature of 

 the air and the water is then equal to 13°. Should we take 



12 glass 



