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 mure 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- 

 ino 1 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. PI. XL) 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 



Vol. VIII. P. II. 3 P instrument 



