FROM THE HIGHER ATMOSPHERE. 485 
the axis of the instrument can, by means of a screw acting on 
the limb of a quadrant, be depressed or elevated to any given 
angle. But the effect will chiefly be produced by the direct 
impressions: for the lateral pulses, striking less obliquely 
against the cavity of the spheroid, will be feebly reflected. 
This moveable axthrioscope was placed in a convenient si- 
tuation out of doors, when the sky appeared free from clouds, 
and had assumed a clear blue tint. The spheroid being turn- 
ed first upright, the effect was noted ; but this continued still 
unchanged, on depressing the axis successively, till it had ap- 
proached the limit of energetic range, or within 20 degrees of 
the horizon. 
From every portion of the sky that subtends a given visual 
angle, there is hence received the same quantity of the frigorific 
pulses. But such would likewise be the result, if they were 
showered from the horizontal surfaces of the successive strata 
which divide the atmosphere ; since, although the intensity di- 
minishes in the ratio of the sine of obliquity, a propellent space 
broader in proportion is, for each elemental angle, brought 
into action, as appears from the inspection of Fig. 10. Let 
BC represent the boundary from which hot or cold pulses 
are darted: A being the point at which these are recei- 
ved, let BS and Ce denote minute portions of the vibrating 
surface. A perpendicular Ck to AC would send impres- 
sions in the direction CA, equal to those emitted by Ce. But 
that the effects produced at A, from the projections of Ck and 
Bd should be equal, the distances CA and BA must be pro- 
portional to those breadths ; wherefore the triangles CA & and 
BA 6 are similar to the elemental angles CA c and BA d equal. 
It hence follows, that the impressions sent from a cluster of 
such angles, amounting perhaps to ten or fifteen degrees, near 
the vertical position, are equal to those contained within the. 
same aggregate angle in an oblique direction. This entire 
agreement : 
