ON PRUNING TIMBER TREES. 255 



past neglect, by the lopping off of large limbs, the places of which 

 the tree now wants vigour to heal. Vast number of trees are 

 destroyed by this system of mutilation, when all further object 

 in pruning is at an end. Sometimes a large branch may be loped 

 off a tree top heavy, or when a branch is likely to be split, or 

 for some other good reason. But it is an error which must end 

 in disappointment, to begin this system of lopping a full grown 

 tree, with the design of compelling it to resume its process of 

 increase when it has naturally ceased. 



When a tree has naturally been neglected, but is not yet so far 

 advanced, but that we may hope to restore it, we have merely 

 to apply the principle of pruning explained to the case of the par- 

 ticular tree. We have to shorten the lateral branches which are 

 forming forks, so as gradually to produce the upright tendency 

 of the leading stem required. The rule is to proceed with the 

 greatest temperance, taking care never to do too much in one 

 season, lest, by depriving the tree of its branches, we enfeeble 

 its vigour and impede its growth. 



ARTICLE V. 



ON THE CLIMATE OF HOT HOUSES. 



BY PROFESSOR DANIELL, KING'S COLLEGE, LONDON. 



The principal considerations which generally guide the manage- 

 ment of gardeners in this delicate department, are those of 

 temperature; but there are others, regarding moisture, which are, 

 I conceive, of at least equal importance. The inhabitants of the 

 hot-house are all natives of the torrid zone, and the climate of 

 that region is not only distinguished by an unvarying high degree 

 of heat, but also by a very vaporous atmosphere. Captain Sabine, 

 in his Meteorological Researches between the tropics, rarely 

 found, at the hottest period of the day, so great a difference as 

 ten degrees between the temperature of the air and the dew- 

 point ; making the degree of saturation about 730, but most fre- 

 quently 5o. or 850 ; and the mean saturation of the air could not 

 have been below 910. Now, I believe, that if the hygrometer 

 were consulted, it would be no uncommon thing to find in hot- 

 houses, as at present managed, a difference of 20o between the 

 point of condensation and the air, or a degree of moisture falling 



