FROM TITE 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 sethrioscope 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 frigorinc 

 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 Bb and Cc denote minute portions of the vibrating 

 surface. A perpendicular C k to AC would send impres- 

 sions in the direction CA, equal to those emitted by C c. But 

 that the effects produced at A, from the projections of C k and 

 Bb should be equal, the distances CA and BA must be pro- 

 portional to those breadths ; wherefore the triangles CA k and 

 BA b are similar to the elemental angles CA c and BA b 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 



