FROM 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, ifthe athrioscope 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 zthrioscope, 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 
eethrioscopical observation performed at the stern or the prow 
of a ship while under full sail. 
In the Pendant Atthrioscopes, 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 
