20 



ting of the wood for some depth is now and then observed on the 

 trunk along the path of discharge. Sometimes these earth dis- 

 charges kill the tree, but in most cases only a disfigurement occurs, 

 which, however, may be great enough to destroy its symmetry. A 

 very much larger number of trees than people are aware of show 

 earth discharges. In most cases of this kind the discharge affects one 

 or more limbs, as the current seldom follows up the main trunk but 

 discharges at the points of several branches. MacDougal* mentions 

 some trees which appear to have been injured by earth discharges. 

 Whether the chemical composition of the soil has any particular 

 bearing upon earth discharges is not positively known. It is known, 

 however, that there frequently exists great differences in the electri- 

 cal potential between the earth and air during thunderstorms and 

 that the electrical conditions of the clouds and earth may change 

 instantly from negative to positive. Some observations made by 

 Mr. A. C. Monahan in our laboratory with a Thomson self-recording 

 quadrant electrometer and water dripping collector show that the 

 electrical potential of the atmosphere varies from a negative charge 

 of 75 volts to 300 positive at various times, at a distance of 30 feet 

 from the ground, and it is also known that trees occasionally dis- 

 charge sparks at their apices, showing that insignificant earth dis- 

 charges occur through trees. 



SUMMARY. 



The adverse conditions with which shade trees have to contend, 

 in cities and towns, constitute a serious drawback to their 

 development. 



A considerable amount of damage occurs to shade trees by wires, 

 causing abrasions, destruction of limbs and leaders, burnings, and 

 necessitating much injudicious pruning. 



The greatest amount of damage caused to trees by alternating and 

 direct currents is by local burnings. The higher the electro motive 

 force (voltage), the more injury is likely to occur to trees. 



There is practically little or no leakage from wires during dry 

 weather. In wet weather, however, when a film of water is formed 

 on the bark, more or less leakage is likely to occur, and if the 



*( Journal of the N. Y, Bot. Gardens, Vol. Ill, No. 31, July, 1902.) 



