246 PROTECTION OF WOODLANDS. 



smoke into sulphuric acid for economical purposes were found to 

 have many technical difficulties to contend with, whilst only one- 

 quarter to one-third of the injurious gas could be withdrawn from 

 the air ; tall chimneys constructed with the intention of conduct- 

 ing the impurities into higher layers of air were practically found 

 to yield no improved results, and occasionally only widened the 

 area over which the injurious effects were noticeable. 



Hence in many places it became the duty of sylviculturists to 

 give some consideration to the matter, so as to minimise the bad 

 effects as far as possible, and in order to prevent the total destruc- 

 tion of the woodlands already more or less damaged. Where areas 

 had already been cleared by the action of the smoke or gaseous 

 impurities, reproductive measures have usually been found to be 

 practically useless efforts, but wherever the injuries have not 

 proceeded quite so far, endeavours should be made to grow the 

 hardier species of trees so as to form protective belts ; these 

 should then never be cleared away, but should only be utilised to 

 a minor extent by the selection of a mature stem here and there. 

 Where the various other physical conditions of soil and situation 

 permit of it, the formation of Oak-coppice is perhaps the best 

 method of utilising the ground, for Oak and Coppice have each in 

 their own way proved themselves specially hardy in regard to 

 injuries caused by atmospheric impurities. 



EXPLANATION OF PLATES. 



I = Imago or perfect insect. 

 P = Pupa or chrysalis. 

 C = Cocoon. 



L = Larva or caterpillar. 

 0=0vum or egg. 



