138 APPLICATION OF PRINCIPLES. 



gravel is, with respect to vegetation, a better coating 

 for the surface of the ground than turf ; it has its dis- 

 advantages as well as its advantages, and the former 

 probably outweigh the latter. In superior heating 

 power is its only advantage ; the objections to it are, 

 its dryness in summer, and its comparative imper- 

 meability to rain, so that it causes the force of per- 

 spiration to be inversely as the absorbing power of 

 the roots. 



It is well known that blackened surfaces absorb 

 heat much more than those of any other colour ; and 

 it has been expected that the effect of blackening gar- 

 den walls, on which fruit trees are trained, would be 

 to accelerate the maturation of the fruit ; but notwith- 

 standing a few cases of apparent advantage, one of 

 which, of the Vine, is mentioned in the Horticultural 

 Transactions, vol. iii. p. 330, this has been, in general, 

 found either not to happen at all, or to so small an 

 extent as not to be worth the trouble. It is true, that 

 so long as the wall is so little covered by the 

 branches and leaves of a plant, the absorbent power 

 of the blackened surface is brought into play ; but 

 this effect is lost as soon as the well becomes covered 

 with foliage. In the early spring, however, before 

 the leaves appear, the flowers are brought rather more 

 forward than would otherwise be the case ; and in the 

 autumn the wood certainly becomes more completely 

 ripened, a result of infinite consequence in the north- 

 ern parts of the country. 



It is rather to a judicious choice of soil and situa- 

 tion that the gardener must look for the means of 



