196 



Messrs. Carpenter and Jeffreys on [Dec. 8, 



thus be considered as representing the mean temperature of the Earth in 

 that region, slightly raised, perhaps, by a downward convection of heat from 

 the surface in the manner to be presently described (§ 90). With such an 

 allowance it corresponds closely with the determinations of the mean tempe- 

 rature of the Crust of the Earth, made by sinking Thermometers into the 

 ground to such a depth as to seclude them from the direct influence of Summer 

 heat or Winter cold, but not to bring them within the direct influence of 

 the Internal Heat of the earth. Thus Quetelet found that a Thermometer 

 sunk to a depth of 24 feet at Brussels showed an annual average of 53°'4, 

 the range of variation being only 2°*5 ; and Bischoff found a Thermometer 

 sunk to a depth of 36 feet at Bonn give an annual average of 51°, with a 

 range of only 1°*5. The Temperature of deep Caves gives another set of 

 data of the like kind, which accord very closely with the foregoing. 

 Thus we have been informed by Mr. Pengelly that the temperature in the 

 part of Kent's Hole at Torquay which is furthest from its entrance varies 

 but little from 52° throughout the year. There is a cave in the island of 

 Pantellaria, lying between Sicily and the African Coast, which is reputed 

 to be of "icy coldness;" but Lieut. Millard, of H.M.S. 'Newport,* who 

 has lately been making a careful survey of the Island, informed us that, 

 although he felt it "very cold'* on passing into it out of a very hot sun- 

 shine, its actual temperature, taken by Thermometer, was 54°. And 

 we have also learned on good authority that this is the temperature of 

 the bottom of the deepest tanks in which water is stored up in Malta, 

 provided that these are excavated (as is very commonly the case) be- 

 neath the houses, or are in any other way secluded from the direct rays of 

 the sun. 



90. Now let it be supposed that the superficial stratum of the water of 

 the Mediterranean had been cooled down by a severe winter to the uniform 

 temperature of the depths below; we have to enquire in what manner 

 it would be affected by the heating-power of the summer sun. This, it is 

 obvious, can be only exerted directly upon the actual surface; for the 

 conducting-Tpower of water is so small that very little downward trans- 

 mission of heat would take place through its agency. Further, as the 

 application of heat at the surface will render the superficial layer specifically 

 lighter, no such convection will take place in the downward direction as 

 takes place upwards when heat is applied at the bottom. But another 

 agency comes into play in the case of Sea-water. The rapid evaporation 

 produced by powerful solar radiation, especially when aided by the hot dry 

 winds of Africa, occasions such a concentration of the surface-film, that, in 

 spite of its elevation of temperature, it becomes specifically heavier, and 

 descends, — to be replaced by a fresh layer. In this manner it will carry 

 down an excess of heat, which diffuses itself through the subjacent layer, 

 of course producing the greatest elevation of temperature in the stratum 

 nearest the surface. The continual repetition of this process through the 

 hot season will carry the elevation of temperature further and further 



