70 The Hygrometer. 



of the stem, and the differences of weight being taken from 

 the counterpoise, the instrument will sink. 



If it is wished to begin at a specific gravity different from 

 water, suppose at 1 "2, for heavy solutions, the only difference 

 will be in loading the instrument. Thus a hydrometer that 

 displaces 300 grains of water, must be loaded to weigh 360 

 grains, that it may stand at the same mark in a fluid whose 

 specific gravity is 1*2. The specific gravity of a body is equal 



360 



300 



any other specific gravity, suppose 1'25, — ■ = — - = 288, and 



to its weight divided by its volume -^ = 1 "2, then to find 



360 



y i-25 



300 — 288 = 12. Twelve grains being added to the counter- 

 poise, the instrument will displace only 288, and -^ == 1*25. 

 — Dublin Phil. Journ. 



THE HYGROMETER. 



Mr. Daniell's hygrometer for ascertaining the dew-point, or 

 the temperature at which air deposits moisture, is the first in- 

 strument which has given precision to this branch of philo- 

 sophy. It is however extremely complex, and requires mani- 

 pulation, which is a heavy objection to any instrument. It 

 admits a double simplification : 



1st. Let the bulb of the thermometer 

 be inserted in a dark glass bottle of 

 aether, and let the surface of this bottle 

 be the surface of deposition. 



2dly. Instead of pouring out aether 

 upon a bulb to cool it, let the bottle of 

 tether itself cool by merely taking out 

 the stopple, so as to allow its evapora- 

 tion. The instrument will take any form, 

 nearly as here represented. 



I would suggest to meteorologists the necessity of an in- 

 strument to show the rate of evaporation at any given time. 

 Mr. Daniell takes for granted, when the temperature is the 

 same, and the dew-point the same, that the bottle of aether 

 would always cool to the dew-point, in the same time measured 

 by a stop-watch. This is by no means certain*. E. F. B. 



* We have inserted this brief notice, because, at a period of science like 

 the present, when the principles of hygrometry and of hygrometers are so 

 much discussed, every opportunity should be allowed for the elucidation of 

 the subject. We have strong doubts, however, of the eligibility of the mo- 

 difications in Mr. Daniell's instrument, recommended by our correspondent. 

 And with respect to his concluding suggestion, we would ask, What other 

 circumstances are essentially concerned in the cooling to the dew-point, 

 beside the temperature and the dew-point itself? — Edit. 



DR. 



