1848.] Observations made on a Botanical Excursion. 385 



Hence the difference between the heat of the surface of the alluvium 

 and of the same at 5 inches is, 16° during the day, but of a similarly- 

 disposed column of sand, 30°. 



During the night again a column of 28 inches of alluvium presents a 

 difference of 17°. 5, one of sand as nearly as I could ascertain of 16 

 inches, 24°. 5. 



This effect of sandy deserts in causing extremes of heat during the 

 day, and cold at night, is thus readily to be apprehended, and in the 

 case of the larger area covered with sand, the effect of radiation is pro- 

 bably much increased. Thus in the desert between Cairo and Suez 

 a surface heated in the middle of December to 90° during the day, 

 presented on the following morning, before sunrise, a dewed surface of 

 47o.5, the increment of heat in digging down to 10 inches was 9 

 degrees : so powerful is then the effect of nocturnal radiation, that a 

 column of 10 inches was cooled at its base to within 9 degrees of its 

 exposed surface ; while a similar one on the Soane had its base temper- 

 ature 24° above that of the surface, &c. 



Observing the flowing sap of a vigorous Calotropis plant growing in 

 the sand to maintain a temperature of 72° in spite of the great heat 

 of the surrounding soil, I dug about its roots and obtained that temper- 

 ature at 78 inches where the sand was wet, and from whence its roots 

 derived their moisture. As at 15 inches the temperature was still only 

 72° and its roots did not appear to descend so deep, it is evident that 

 the plant was pumping up moisture with such rapidity as to bring the 

 fluid to the surface as cool as below. That this coolness of the sap is 

 due to the ascending currents, is proved by taking the temperature of 

 the leaves, which were at 80° (constants). 



The low temperature of the leaves exposed to the sun (which heated 

 the sand to 110° and earth to 104°) is probably due both to the coolness 

 of the ascending sap and evaporation from the leaf's surface, as the 

 activity of the circulation is regulated by the rapidity of evaporation. 

 On the same night the leaves were cooled to 54° by radiation, the sand 

 to 51°, and before sunrise on the following morning the Calotropis 

 showed 45°. 5 and the sand 42°. I neglected to observe the temper- 

 ature of the sap at this time, but supposing it to be that of the earth 

 at the same depth (15 inches) which was 68°, we must admit the leaves 

 to be heated only 8° by solar radiation and cooled 2 2°. 5 by nocturnal. 



