indicating Means for its Prevention. }5$ 



till about 50 or 80 feet : beyond that the temperature of the 

 earth is invariably found to increase, and is supposed to do 

 •so continually as we approach the centre of the earth. 



The warmth of the sub-stratum of mould in gardens, or 

 wherever the surface or vicinity abounds with the roots of 

 • vegetables, may, I think, be reasonably expected to exceed, 

 at times, the temperature of situations not so circumstanced : 

 for, as the roots of vegetables have the power of absorbing 

 moisture from their vicinity, and this having ever a ten- 

 dency to penetrate toward that point where may prevail the 

 greatest dryness ; so, in proportion as these roots exhaust the 

 moisture of the earth immediately surrounding their fibres, 

 will the aqueous vapour from greater depths be attracted *. 



Hence, whenever extreme drought in the soil and super- 

 natant atmosphere is such as to be unfavourable to vegeta- 

 tion, the finely divided aqueous vapour from beneath will 

 ascend toward the upper vegetable mould ; where having 

 penetrated to that level which (owing to the non-conduct- 

 ing quality of the earth) can only, as we have seen, be af- 

 fected by the temperature of the. air 5 namely, the uppermost 

 stratum of soil within a foot or two of the surface, .the va- 

 pour so arising must undergo a condensation agreeable to 

 ~the established laws of heat, w hereby the before combined 

 caloric of that vapour, now rendered free, becomes diffused 

 among the surrounding media, augmenting of course the 

 temperature of the adjacent earth, and that as frequently in 

 summer as at other times f . Consequently, wherever there 

 exists the most frequent cause of dryness in a soil, there 

 will the rise and condensation of subterranean aqueous va- 

 pour be most frequently found to -ensue; and agreeable 

 thereto, the effect must more repeatedly take. place in situa- 

 tions where vegetables abound than elsewhere. 



On the contrary, in humid states of the atmosphere, this 

 water for the. support of the vegetable kingdom will be by 

 priority attracted from the air (dry earth acting as an hy- 

 grometer), and the effect will be, the soil cannot then have 

 that warmth imparted to it as formerly, from the two-fold 

 cause of the moisture of the atmosphere being at a lower 



* In the driest season, if a drinking glass be placed on the ground in 

 an inverted position, its interior surface will soon be seen covered with 

 drop of w;ucr, from the condensation of these extremeiy divided vapours 

 exhaled from the earth, and condensed on the cold sides of the glass. TJie 

 fluid thus evaporated from the earth, Watson and Hales have calcuimd 

 to amount to a consideiable quantity per hour from a square foot, of 

 ground, even after a drought of some weeks' continuance'. 



f Hales's Staticks, vol. i. p. fiy. 



M 3 temperature 



