'with regard to the Cnltivatioji of Exotics. 15 



are objectionable; and many have experienced their ill effects. 

 Those who have houses to erect would, therefore, do well to 

 turn their attention to a thorough investigation of this subject, 

 on which the result of one slight calculation may incline them 

 to enter more seriously. Supposing, for the sake of easy num- 

 bers, one house to be 20 ft. high, and 20 ft. wide; and another 

 to be soft, high, and only 10ft. wide: the contents of the for- 

 mer are exactly double those of the latter ; and, at the same 

 time, instead of containing double the surface of glass in its 

 roof, it has scarcely one third more; being nearly in the pro- 

 portion of 28 for the house of double volume, to, not 14, but 

 22, for the one of half the internal capacitj". In the wide house, 

 every square foot of glass has to heat upwards of 7 cubic feet 

 of air; in the narrow house, about 4|;ft. 



In what may be termed the higher department of forcing, 

 there are, perhaps, fewer objectionable points than in the lower 

 scale; where the plants are not kept in pots, and are conse- 

 quently liable to great discrepancy between their terrestrial and 

 atmospheric temperatures. Much has been written on the latter; 

 and in practice it has been found best to approximate it as nearly 

 as possible to that climate in which the given species of exotic 

 plants naturally acquires the highest perfection. Beyond this, 

 nothing need or can be advanced ; but, with regard to what may 

 be termed terrestrial temperature, something useful may be 

 stated. The vine, for instance, as regards temperature, may be, 

 and often is, so situated as to have its shoots in the climate of 

 Syria, whilst at the same time its roots are in that of Britain. 

 Such being the case, there need be less surprise at the ill success 

 which occasionally accompanies its cultivation, than at the reputed 

 inexplicable causes of bad setting, shriveling, and shanking of 

 the grapes. The mean temperature of the soil, or that portion 

 of the earth extending to several feet below its surface, is nearly 

 the same as the mean temperature of the incumbent atmosphere. 

 The soil of England, so far as the generality of roots penetrate, 

 may therefore be estimated at about 50° Fahr, for the average. 

 In Armenia and Syria (which may be reckoned the native 

 region of the vine, for there, since the remotest accounts of 

 history, it has felt itself at home,) the mean temperature of the 

 soil will not be below 60° ; and in the growing season its tem- 

 perature will, doubtless, be above 70°, corresponding with the 

 temperature of the atmosphere which is imitated in the vineries 

 of Britain ; but forming a great discrepancy with the temperature 

 of the soil of this country. But, although this will be allowed 

 to be bad, still the worst of the evil remains to be noticed : the 

 above is only a medium case of general occurrence ; and, although 

 some may not be so widely different as 10° in the action of tem- 

 perature on root and branch, yet there are, in all probability, 



