ELECTRICAL INJURIES TO TREES. 15 



The potential of the air is usually negative, although occasionally 

 changing to positive. In the ease of coniferous trees, however, like 

 the Norway spruce,^ we found that the potential under the foliage was 

 invariably positive or similar to that of the earth, which may be ex- 

 plained on the theory that conifers are constantly discharging positive 

 electricity to such an extent that the air suiTounding them becomes 

 charged similar to the earth. To what extent the film of water on the 

 bark is capable of equalizing the difference in electrical potential in 

 the air surrounding the trees, as well as the ground and in the tissues 

 themselves, has not been wholly determined, but we had difficulty in 

 obtaining potential readings under the foliage of elms in wet weather 

 in our exiDeriments covering two summers. This may in part be ex- 

 plained by the improper installation of our collector. It is not un- 

 likely that the film of water on the bark of trees during such periods 

 would have a tendency to affect materially the potential of the sur- 

 rounding air, and possibly to equalize the electrical tension. The sub- 

 ject should have further investigation, but we believe that it is possible 

 to protect trees from injury by lightning, whether they be atmospheric 

 or earth discharges. 



Methods of preventing Injury to Trees from Wires. 



The constantly increasing use of electricity for various purposes 

 makes necessary a more extensive use of wires which have become a 

 great menace to shade trees. The appearance of streets is also hardly 

 improved by the increased number of poles and wires, and the legal 

 restrictions as to the height, distance apart, etc., of the wires of the 

 telephone, telegTaph, trolley and electric light companies make the 

 problem of maintaining shade trees on the same street with public- 

 service corporations a serious one. Of all the troubles with which tree 

 wardens have to contend the wire problem is often regarded as the worst. 

 Notwithstanding the strict laws which some States have adopted in re- 

 gard to injuring shade trees, the agents of some public-service corpo- 

 rations often nave little regard for trees or the laws respecting them. 

 Where 40-foot poles must carry the wires of three or four public- 

 service corporations there can be little or no opportunity to preserve 

 the natural symmetiy of shade trees, esjjecially when low branching 

 maples and other trees are planted on the same side of the street with 

 the wires. There is less interference from limbs with low than with 

 high-tension wires. Trees like the elm, whose branches form acute 

 angles, offer less obstruction to wires than maples; but not all streets, 

 of course, are planted with elms, which may be as well, considering 

 their susceptibility to various pests and unfavorable climatic conditions. 



The best solution of the wire problem lies in burying the wires. This 



I Mass. (Hatch) Agr. Exp. Sta. Rept. 1905, p. 14. • • 



