SHADE TREES. 



243 



cloth on a hard surface. The whole tree is not affected as a rule, as the 

 lightning stroke seldom follows up the main trunk, but discharges at the 

 points of several branches. As a rule, however, one side of the trunk and 

 one or more of the limbs on that side are affected and the symmetry of the 

 tree destroj^ed. The first indication of the discharge is shown bj' the 

 immediate wilting and subse- 

 quent death of the leaves of the 

 affected limbs, which also die 

 later. In the course of time cracks 

 similar to those caused by frost, 

 and later ridges due to healing, 

 will be seen on the trunk, show- 

 ing the path of the discharge, 

 and occasionally, when the in- 

 jury is considerable, the bark near 

 the affected part of the tree falls 

 off. The limbs, however, are not 

 always killed, frequently splitting, 

 and a cracking of the wood for 

 some depth is now and then 

 observed on the trunk and limbs 

 along the path of discharge. 



Whether the chemical composi- 

 tion of the soil has an^^ particular 

 bearing on earth discharges is not 

 positively known. It is known, 

 however, that there frequently 

 exist great differences in the elec- 

 trical potential between the earth 

 and air during thunderstorms, and 

 that the electrical conditions of 

 the atmosphere and earth may 

 change instantly from negative to 

 positive. Some observations made 

 in our laboratory with a Thom- 

 son self-recording quadrant elec- 

 trometer show that the electrical 

 potential of the atmosphere, at a 

 distance of 30 feet from the 

 ground, may vary, often in a 



brief period, from a few volts to 300 or more. It is also known that 

 trees occasionally discharge sparks at their apices, showing that insig- 

 nificant earth discharges occur through trees; and when the soil in which 

 potted plants are growing is charged electrostatically, small sparks are 

 thrown off from the leaves. Earth discharges through trees, whether 

 strong or weak, appear to be similar in nature, and may be associated 



Fig. 102. — Maple showing effects of earth 

 discharges (lightning), causing splitting 

 of the trunk and death of limbs. 



