14 MASS. EXPERIMENT STATION BULLETIN 156. 



water-dripping collector show that the electrical potential of the at- 

 mosphere varies from a negative charge of 75 volts to 300 positive 

 at various times at a distance of 30 feet from the ground; and our 

 records show that most of the time the atmosphere is charged posi- 

 tivel5^ It is also known that trees occasionally discharge sparks at 

 their apices, showing that insignificant 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 with changes in the potential of the 

 earth and atmosphere. The liigh electrical resistance shown by plants 

 in general, as already stated, seiTes as a great protection against death 

 from lightning and electric currents. 



Susceptibility of Different Trees to Lightning Stroke. 



There has always been great difference of opinion in regard to the 

 susceptibility and non-susceptibilitj- of various trees to lightning, and 

 the data on the subject gathered from this and that source are alto- 

 gether too meager to admit of reliable statistics. But it is known that 

 the location of the tree, nature of the soil, elevation, etc., are of great 

 importance in determining susceptibility to lightning. 



It has already been pointed out that electrical resistance is influenced 

 by temperature, and the percentage of moisture in the tissues is also 

 an important factor. During thunder showers trees become more or 

 less drenched with rain, and according to Stahl,^ the more thoroughly 

 wet the tree is the less suscej^tible it becomes to lightning stroke. He 

 bases his observations on the fact that smooth-bark trees like the beech 

 and others, which are considered more immune to lightning, become 

 thoroughly wet during storms, while the oak and other rough-bark 

 trees do not. Stahl's idea, therefore, is that smooth-bark trees possess 

 a better water-conducting surface and have a tendency to equalize the 

 electrical tension existing between the atmosphere and the ground, so 

 that they are rendered less susceptible to lightning. His deductions 

 were based upon experiments with electrical discharges made with the 

 bark of different species of trees containing various percentages of 

 moisture. He further observed that vertical limbs were more likely 

 to become drenched than horizontal, and that the lenticels and stomal a 

 play a role in the equalization of the differences in electrical potential 

 existing between the tissues and the atmosphere, etc. There appears 

 to be no difference in the electrical potential under deciduous trees 

 and in the open air when there is no foliage, at corresponding heights,^ 

 and the electrical potential will average 40 per cent, less under the 

 foliage of trees than in the open air when the foliage is developed. 



1 Stahl, E. Die Blitzgefahrdung der verschiedenen Ba\xmarten, Jena, G. Fisher, 1912. 



