4 
FROM THE HIGHER ATMOSPHERE. 481 
pendicular to the axis, and having the sentient ball of the py- 
roscope placed in the lower focus, (see fig. 2, 3. and 4.) It is 
evident that the impressions which pass through the first focus 
in every direction, must be reflected towards the second focus. 
But since the upper focus occupies the middle of the aperture, 
all the cold pulses which enter the spheroid will be nearly col- 
lected on the sentient ball of the pyroscope. The more oblique 
impressions, however, striking the sides of the reflector at a 
greater angle of incidence, will be less copiously reflected ; and 
consequently this compound instrument will indicate more fully 
the action of that quarter of the heavens to which it is turned. 
Of these truncated spheroids, I had two made of planished 
tin; the one being eight inches high, with an extreme width 
of the same, and its focus two inches from the bottom; and 
another nine inches high, and six inches broad.; the focus be- 
ing only an inch from the bottom. I had one or two more 
reflectors, constructed of plated copper, after the same forms, 
but with only half those dimensions. The more eccentric 
spheroid being mounted on pivots, was better fitted to. explore 
any particular portion of the sky, (see fig. 2. Pl. XI); To this 
inquiry my attention. was now drawn. Watching when the sky 
appeared most serene, I directed the instrument, first to the 
zenith, and then. successively declined it, till it came within. 
twenty degrees of the horizon. But the instrument continued 
to indicate the: same frigorific impression, or about forty or 
fifty millesimal degrees: at all inclinations. It was, therefore; 
ascertained, that the action of a given section or angular por- 
tion of the sky, is the same at every obliquity.. 
With the erect spheroid, I found, in cloudy weather, that the: 
frigorific impression diminished, in proportion as the humid 
mass, floating in the atmosphere, seemed to descend. When 
the sky was canopied with high fleecy clouds, the effect on the 
Vor. VIII. P.IL 9 instrument 
