Dr. Cumming'5 New Hygrometer, 



403 



faction with which the labours of Mr. Daniell, as well as those 

 of Mr. Jones, to produce a hygrometer that should readily 

 mark the •* comparative degrees of moisture and dryness in the 

 atmosphere, and, by exhibiting them in degrees of the thermo- 

 meter, refer them to a known standard of comparison, and 

 speak in a language which every body understands," have been 

 received by meteorologists. 



Of the merits of the hygrometers of Messrs. Daniell and 

 Jones, it would ill become the author to speak, as an accident 

 upon the road, after waiting with impatience for months for the 

 arrival of Mr. Jones's instrument, deprived him of the power 

 of instituting a set of comparative experiments ; but should 

 Mr. Daniell's observations upon Mr. Jones's hygrometer, as 

 published in No. XLII. of the ** Journal of Science and the 

 Arts," prove correct, the accident above alluded to may fairly 

 be considered the efficient cause of the production of the follow- 

 ing improved instrument. 



The hygrometer in question is extremely simple, 

 and is intimately founded on the principle pur- 

 sued by Mr. Dalton in his mode of interrogating 

 Nature. But instead of using ** a tall cylindrical 

 glass jar, with water fresh from the well, and artifi- 

 cial cold mixtures," I employ the accelerated eva- 

 poration of aether, or of a less costly article^ recti- 

 fied spirit of wine, by directing a current of air 

 through a thin glass or metallic tube, of convenient 

 size, containing a delicate thermometer enveloped in 

 sponge or other porous substance : when it will be 

 obvious that, by the affusion of a little aether upon the 

 porous body surrounding the bulb of the thermo- 

 meter, almost any degree of artificial cold may be 

 produced by simply mounting the prepared tube 

 upon the nozzle of a small pair of bellows, or other 

 pneumatic contrivance ; thus rendering, by the ab- 

 straction of heat from the tube employed, the con- 

 densation of aqueous vapour upon its exterior sur- 

 face, a matter of ease, certainty, and expedition, 

 while the comparison of the temperature of the air, at the com- 

 mencement of the experiment, with the mean of the indications 



