480 ON IMPRESSIONS OF COLD 



weather being very unsteady, I had to watch the intervals of an 

 opening sky. Covering the mouth of the pitcher with a pane 

 of glass, the pyroscope would sometimes indicate one or two 

 degrees of heat ; but, on removing that screen, it always mark- 

 ed impressions of cold amounting perhaps to five or six de- 

 grees, as often as the clouds parted, and left a blue space near 

 the zenith. The continual darting, by day and night, of cold pul- 

 sations from the sky was thus ascertained; and nothing seemed 

 wanting but to improve the construction of the instrument, and 

 render it commodious and portable. I resolved to inclose both 

 balls within the same cavity ', the lateral ball being gilt, and of 

 the same colour as that of the annexed reflector; and the naked 

 sentient ball being placed in the focus. By this disposition, 

 the influence of light was almost completely destroyed, its ac- 

 tion being made equal on both the balls. But, to protect those 

 balls from the disturbing effect of wind, it was requisite, that 

 the cup containing them should be deep and rather nar- 

 row. If the cold pulses to be measured were darted only in a 

 vertical direction, the parabolic conoid would certainly be the 

 proper form of the reflector. I was convinced, however, from 

 some imperfect trials, that such impressions were likewise sent 

 down at different angles of obliquity. Supposing their inten- 

 sity to be equal in all directions, an hemispherical reflector would 

 answer best. But the focus, instead of a single point, would then 

 have branched into the diverging arcs of the caustic curve, and 

 the sentient ball of the pyroscope would have required half the 

 dimension of the reflector itself, (see fig. 11. PI. XL) The main 

 object, however, was to measure the impressions received from 

 the upper portion of the atmosphere, that part near the horizon 

 beino- generally obstructed by clouds or vapour. I therefore a- 

 dopted an intermediate plan, and selected for the reflector a trun- 

 cated oblong spheroid, cut through the upper focus by a plane per- 

 pendicular 



