1847.] Bottom Heat. 335 



feet in ice. The manner of heating that has been adopted at 

 Bayswater has the fault oi being too expensive. Let Polmaise 

 be substituted for it, and the whole cost will consist in forming a 

 cavity in which the warm moist air shall circulate, when desired. 



The necessity of providing some means of warming the borders 

 of Vines to be forced (we will even say grown) in a climate like 

 this will be obvious when we compare the temperature of the 

 earth in the south of France and Great Britain. The mean tem- 

 perature of the earth, near London, in the three first months of 

 the year, may be taken at 38 deg. ; that of Marseilles or Bor- 

 deaux will be at least 65 deg. The mean temperature of the 

 earth near London, in July and August, is 62 deg.; that of Mar- 

 seilles about 78 deg., and of Bordeaux 77 deg. We w^ill ask 

 whether it is probable that such differences in the soil can be un- 

 important to the plants which grow in it. It would be a capital 

 experiment to attempt to grow Grapes in a house whose border 

 should be in the inside the house, and into which no other arti- 

 ficial source of heat should be admitted. 



We have little idea in this part of the world of the temperature 

 of the soil in some countries; Captain Newbold found the heat of 

 the granitic soil in the vicinity of Beltary, at 2 P. M. in May, as 

 high as 121 deg.; that of the black soil, 122 deg. 5 min.; that of 

 the air in the shade being 95 deg. 5 min. At midnight the tem- 

 perature of the black soil was still as high as 86 deg. that of the 

 air being 80 deg. That of a bare rock of granite, in the same 

 locality, at 2 P. M., w^as 120 deg. 5 min. of black basaltic rock, 

 122 deg.; that of the granite at midnight was 86 deg. 5 min. 

 Other examples are given in the " Theory of Horticulture." 



But in attempting to apply these principles to practice garden- 

 ers are stopped at the threshhold of their inquiry by the absence 

 of evidence as to the temperature of soil in different countries. 

 By a persevering search though books, they find, indeed, plenty 

 of statements as to the temperature of the air, but that of the earth 

 observers have almost invariably neglected. It is, therefore, in- 

 teresting to inquire whether the temperature of the earth in which 

 plants grow may not be inferred from that of the air in w^iich 

 rests upon the suri'ace. It has been shown, in the " Theory of 

 Horticulture" (p. 96), that in October, near London, the mean 

 temperature of the earth has been found 3 deg. or 4 deg. above 

 that of the air, although in general the difference is not more than 

 a degree or a degree and a half in favor of the earth. The per- 

 manent heat of the earth may, therefore, be regarded as being al- 

 ways higher than the mean of the air; but the amonnt of differ- 

 ence will be regulated by the temperature to which the earth is 

 exposed, and by its own conducting qualities. It seems to us, 

 however, that for gardening purposes, the temperature of the 



