492 ON IMPRESSIONS OF COLD 



pointed to the zenith, marked only five millesimal degrees ; 

 but, on lowering it successively to the angle of 30 degrees 

 above the horizon, it continued to indicate still the same effect. 

 Water almost completely absorbs the pulsatory impressions of 

 heat or cold ; and may not clouds, consisting of diffuse aque- 

 ous particles, produce a similar effect ? But the feeble action 

 of five degrees, amounting scarcely to the eight part of what is 

 observed in clear weather, could not be any remnant of the 

 pulses from the higher celestial regions, which had penetrated 

 through the mass of vapours ; because, if the vertical transit, 

 through the obstructing range, allowed only an eighth part to 

 escape, the oblique passage of 30 degrees, redoubling the ex- 

 tent of absorption, would have reduced the final discharge to 

 five-eighths of a degree. The impression measured by the 

 sethrioscope, in this case, must therefore have originated wholly 

 in the strata of air between the under surface of the clouds 

 and the ground. But in that narrow space, the extreme diffe- 

 rence of temperature would be comparatively small. Hence 

 the frio-orific action is found always to diminish as the clouds 

 descend. Nor does their variable denseness appear materially 

 to affect the result, which is often the least, when a very thin, 

 whitish, but low vapour, gathers in the atmosphere. Hence 

 the gethrioscope might, with great facility, be employed in es- 

 timating the altitude of clouds. 



As the higher strata of the atmosphere thus dart cold pulses 

 downwards, so the lower strata must evidently project equal 

 pulses of heat upwards. But to measure these, it would re- 

 quire, as in fig. 7. the aethrioscope to be inverted and furnished 

 with a pendant differential thermometer. The instrument, now 

 carried to the top of a lofty mountain, and directed to the plain 

 below, would indicate a considerable impression of heat, nearly 

 proportional to the quantity of ascent ; and, therefore, amount- 

 ing, for example, on the summit of Chimboraco, to perhaps 20 



millesimal 



