SHADE TREES. 239 



become intensely heated, destroying the living tissues and girdling the 

 tree to a considerable distance. The part of the trunk towards the rail is 

 almost invariably the most severely affected. With reversed polarity, as 

 already pointed out, the injury is confined mainly to the base of the 

 trunk, where the destruction of tissues causes great damage. Such dam- 

 age does not occur when a positive overhead feed wire comes into con- 

 tact with limbs. The entire area between the base of the tree and the 

 overhead wire is not, as a rule, affected, although the extent of injury may 

 vary somewhat. On the elm shown in Fig. 99 the burning was caused by 

 a reversed system, and there was only slight injury at the point of contact 

 with the overhead wire, while at the base about 6 or 7 feet of the tree were 

 affected. This injury takes place when the soil and the bark of the tree are 

 moist, and may occur during a single period of excessive moisture, or 

 intermittently. In some instances trees show serious effects a short time 

 after the current has been reversed, when the bark will become loose and 

 later fall off. The writer has observed both elms and maples some of 

 them 2 feet or more in diameter which have been killed in this way. 

 In some cases the trees were not more than 3 feet from the rails, while in 

 others the distance was considerably greater. 



In one city, 51 trees were reported killed or so badly injured as to be 

 of no value, 67 had large limbs removed, and many more were saved by 

 removing limbs likely to come into contact with wires. Some of the in- 

 jury took place on streets having trolley wires but no electric railways, 

 and it is surmised that the ground connections were made through several 

 pipe lines located near the trees, which led very close to the electric rail- 

 way. According to Mr. G. A. Cromie, 1 who had these under observation, 

 the injured trees were in some cases located from 200 to 1,000 feet from 

 the track. The effects on the trees were noted shortly after the street 

 railway had changed its system, i.e., using the rail to carry the positive 

 and the overhead wire the negative or return current. The trees in con- 

 tact with the overhead wires became electrically charged, and when wet 

 it was impossible for linemen to work on them. Under these conditions 

 the insulation was much less efficient, and even wooden sleeves im- 

 bedded in coal tar and rubber proved of small use in preventing leakage. 

 A large percentage showed a characteristic burning at the base, and 

 the bark was burned off in some instances to quite an extent. One limb 

 that had been in contact with the negative feed wire was found dead, but 

 the tissue at the base of the trunk was normal. Dr. J. W. Tourney, 

 director of the Yale Forestry School, who examined many of these trees, 

 found a disintegration of the wire where it came into contact with the 

 limbs, apparently due to electrolytic action, and chemical analysis showed 

 the presence of copper and zinc in the tissues of the wood that had been 

 in contact with the negative or overhead wire. Dr. Tourney believes 

 that in such cases the disintegration of the copper wire and the absorption 

 of the copper by the tissue were responsible for the death of the limbs. If 



1 G. A. Cromie, "Scientific American" supplement, No. 1985, p. 40, Jan. 17, 1914. 



