476 



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 mater- 

 nally 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, is encircled 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 



* 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. 



