PllOM THE HIGHER ATMOSPHERE. 483 



and endeared to me by many pleasing associations. The re- 

 sult of them has been the construction of a delicate instrument, 

 which will be deemed, I hope, a valuable accession to meteo- 

 rology, and indeed to physical science in general. From the 

 term ai&^iog, which, in reference to the atmosphere, signi- 

 fies at once clear, drif and cold, I have appropriated the 

 name of JEthrioscope to this new combination of the py- 

 roscope. The sensibility of the instrument is very striking, 

 for the liquor incessantly falls and rises in the stem with 

 every passing cloud. Under a fine blue sky, it will some- 

 times indicate a cold of 50 millesimal degrees ; yet on other 

 days, when the air seems equally bright, the effect is only 30°. 

 The causes of these variations are not quite ascertained. The 

 action is in general greatest under a clear and translucid at- 

 mosphere. But particular winds,, blowing at different altitudes, 

 seem to modify the effect, and so may perhaps the transition- 

 from summer to winter* 



There are three principal forms of the iEthrioscope : 1. Erect; 

 2. Sectoral ; 3. Pendant. 



1. Erect. — All the requisite conditions for measuring the at- 

 mospheric impressions are attained, by adapting the pyroscope 

 to the cavity of a polished metallic cup, of rather an oblong 

 spheroidal shape, the axis having a vertical position, the lower 

 focus being occupied by the sentient ball, while the section of a 

 horizontal plane, at the upper focus, forms the orifice, (see 

 fig, 3. PI. XL) The cup may be made of thin brass or sil- 

 ver, either hammered or cast, and then turned and polish- 

 ed on a lathe ; the diameter being from two to four inches, and 

 the eccentricity of the elliptical figure varied within certain li- 

 mits according to circumstances. The most convenient propor- 

 tion, however, is, that the eccentricity should be equal to half 

 the transverse axis, or that the focus should be placed at the 



3P2 third 



