ON AQUEOUS AND IGNEOUS METEORS. 70^ 



inilar, that even surfaces of different kinds possess different attractive powers 

 for moisture nearly ready to be deposited. It is, however, only necessary to 

 employ, for Mr. Dalton's experiment, a substance whicli has a very weak attrac- 

 tion for moisture; and any kind of metal will perhaps be found sufficiently 

 correct in its indications. 



It has been observed, that a piece of metal, placed on glass, usually protects 

 also the opposite side of the glass from the deposition of dew; and Mr. Bene^ 

 diet Provost has shown, that in general, whenever the metal is placed on the 

 M'armer side of the glass, the humidity is deposited more copiously either on 

 itself, or on the glass near it; that when it is on the colder side, it neither re- 

 ceives the humidity, nor permits its deposition on the glass; but that the ad- 

 dition of a second piece of glass, over the metal, destroys the effect, and a se- 

 cond piece of metal restores it. It appears that, from its properties with 

 respect to radiant heat, the metallic surface produces "these effects, by pre- 

 venting the ready communication either of heat or of cold to the glass. 



The quantity of invisible moisture, contained in air,may be,in some degree, 

 estimated from the indications of hygrometers, although these instruments 

 'have hitherto remained in a state of great imperfection. A sponge,' a quan- 

 tity of caustic potash, or of sulfuric acid, or a stone of a peculiar nature, has 

 sometimes been employed for determining the degree of moisture of the air, 

 from which it acquires a certain augmentation of its weight. A cord dipped 

 in brine, or the beard of an oat, is also often used for the same purpose: the 

 degree in which it untwists, from the effect of moisture, being shown by an in- 

 dex. Buttheextensionofahair,orofaslipof whalebone, which have been employ- 

 ed by Saussure and Deluc, appear to be more certain and accurate in their indi- 

 cations. The hair hygrometer acquires more speedily the degree correspond- 

 ing to any given state of the air, but it seems to reach the utmost extent of 

 its scale before it arrives at perfect humidity ; while the whalebone hygrome- 

 ter appears to express a greater change upon immersion in water than from the 

 effect of the moistest transparent air, which has also been considered by some 

 as an imperfection. Both these instruments are impaired by time, and ac- 

 (juire contrary errors, so that a mean between both is more likely to be cor- 

 rect than either separately. Their indications are at all times widely different 

 from each other, and the mean appears to approach much nearer to a natural 



VOL. I. 4t 



