TROM THE HIGHER ATMOSPHERE. 487 



tion on the sentient ball, scarcely exceeding, however, the tenth 

 part of the entire and original impression. A lid of glass or of 

 mica intercepts the impressions like one of paper ; for the admis- 

 sion of light has no deranging effect, if the eethrioscope be rightly 

 constructed and highly polished. The minute secondary ac- 

 tion is almost extinguished, if screens of paper, glass, or mica, 

 be held at some distance above the mouth of the instrument. 



The variety composed of two hemispherical cups will answer, 

 as an inverted eethrioscope, for measuring, at some elevation, 

 the warm pulses sent up from the lower strata. It is only re- 

 quisite to cover the upper hemisphere during the observation 

 with its metallic lid. The same form of the instrument might 

 likewise conveniently be employed, when its altitude is not 

 very considerable, to determine the difference of the tempera- 

 ture of the surface of the earth, or of the sea, from that of the 

 superincumbent stratum of air. This difference, it would ap- 

 pear, from some unfinished observations which I have made, 

 is expressed, on Fahrenheit's scale, by two-thirds of the millesi- 

 mal degrees indicated by the compound pyroscope. Nor is 

 this effect sensibly altered by the proximity of the terminating 

 surface, because its indefinite expansion will always present 

 nearly the same visual angle. Hence the relative temperature 

 of the surface of the sea, may be easily discovered from an 

 aethrioscopical observation performed at the stern or the prow 

 of a ship while under full sail. 



In the Pendant iEthrioscopes, both the glass balls are left 

 naked ; but, in the Erect and Sectoral kinds, the lateral ball is 

 always gilt. This condition, however, is not essential, since the 

 concentration at the focus would be sufficient, by its excess alone, 

 to produce an adequate effect. Hence the observations may 

 be varied, by introducing within the reflector a differential 

 thermometer, either consisting of translucid balls, or composed 

 of balls blown from black glass. When the latter kind is used, 



the 



