4716 ON IMPRESSIONS OF COLD 
day, therefore, the ground always becomes warmer than the 
incumbent stratum of atmosphere. This: effect is greatest 
in the tropical countries, where the sun gains a higher altitude, 
and pours his light in a full stream. Winds very mate- 
rially check the accumulation of heat at the surface, and the 
calorific action is besides diminished in cloudy weather. A 
ploughed field is more affected by the sun’s rays than a 
grassy plot, since a loose or spongy superstratum, by exposing 
multiplied surfaces, dissipates more quickly the impressions of 
heat communicated to it. 
The best mode of examining the difference between the tem- 
perature of the surface and that of the incumbent air, is by means. 
of a pendant differential thermometer, from one to three feet 
in length. It consists of a ball and long stem, to which ano- 
- ther similar ball, with a short portion of tube, having its bore 
swelled into a narrow cylindrical reservoir, is hermetically join- 
ed. The reservoir exceeding not the tenth of an inch in dia- 
meter, detains and supports the tinged sulphuric acid by its 
capillary attraction, (see fig. 5. Pl. XL) This instrument be- 
ing suspended in a vertical position, the lower ball approach- 
ing or resting on the ground, while the upper ball, at a 
moderate” elevation S, is enci ‘cled by the incumbent stratum 
of air, the rise of the coloured liquor in the stem will mark 
the excess of warmth below, and indicate very minute dif 
ferences of temperature. Last summer I made some ob 
servations of that sort, but not so extensive as I have since 
projected. The effects were found to be extremely various. 
In sun-shine, and calm, weather, the ground was sometimes 
thirty millesimal degrees warmer than the air only a few 
inches above it *. But when the sky happened to be much 
overclouded, or when strong winds swept over the surface, 
the 
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* It may be proper to repeat, that, in all the combinations of the differential 
thermometer, the divisions of the scale are made to correspond with the thousand 
parts of the interval between freezing and boiling water. 
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