DAMAGE TO TREES BY ACID FUMES. 705 



4J miles from the works, but naturally the amount of injury 

 done varies inversely with the distance. Trees bordering on 

 the wood, and especially on woods to the east and north-east 

 of the works, will suffer most. In narrow valleys even the 

 smoke of locomotives has proved prejudicial to trees on either 

 side of a railway. Oliver states that the effects of London 

 fog extend to 35 miles westward, and that seedlings of Cucur- 

 litaceae and tomatoes are thus killed at even that distance 

 from London. 



It has been observed, chiefly in the Oberharz, that woods 

 growing on fertile soil resist acid fumes better than those on 



poor soils. 



e. Climate. 



Exposure to light and moisture are not without influence on 

 the action of acid fumes. Leaves suffer more when dew is 

 resting on them than when they are dry. Thus the damage 

 will be at its maximum after rain at midday, and at its 

 minimum with cool nights and dry days. The damage during 

 rainy weather, though more severe than in dry weather, does 

 not extend far from the works, as the rain speedily dissolves 

 the fumes. 



/. Sundry Circumstances. 



When older woods overshade an underwood, the former may 

 protect the latter from damage by fumes, and trees standing 

 above the general leaf-canopy of the wood, such as standards 

 in high forest or above coppice, suffer most. The shelter 

 afforded to crops and orchards by walls and hedges is also 

 considerable. 



Damage is not so soon marked in young woods under 30 

 years old as in older woods. In old woods, especially coni- 

 ferous, damage is soon recognisable. Their foliage soon 

 becomes thinner ; the shoots dry up, and death soon occurs, 

 often in two or three years. 



4. Methods oj Recognising Damage. 



The question regarding external or internal signs for recog- 

 nising damage by fumes is of great scientific interest. It is 

 also of practical importance, as in the disputes or lawsuits 

 between owners of woodlands and of smelting furnaces, the 



F.P. Z Z 



