100 
Annual Report of the 
on or near the surcace, Prof. Henry has shown by a very ingenious 
experiment. 
This fact has an important bearing on the construction of light¬ 
ning-rods. If the rod be cylindrical, the fluid in its passage will be 
equally distributed over the whole surface, and its capacity will be 
as its surface or diameter. This will not be true of a rod of any 
other form. If, for example, the rod be grooved, the fluid will on ac¬ 
count of the mutual repulsion of its particles tend to pass along 
that part of the surface most remote from the centre or axis of the 
rod, so that its capacity will not be as its actual surface. Besides, 
there will be a tendancy of the fluid to pass off explosively from 
any part of the surface approaching towards a sharp edge. 
3. The passage of a discharge through any medium, as air, im¬ 
parts to its particles a repulsive force which causes them to sepa¬ 
rate with explosive energy. The mechanical effects produced by 
lightning are due principally to this cause. A powerful discharge of 
electricity through a confined portion of air has been known to 
impart such repulsive energy to the particles of air as to lift the 
roof from a building. The splintering of trees by lightning and 
other mechanical effects of this kind, are to be explained in the same 
way. 
4. When an explosive discharge takes place between two bodies, 
the greatest energy is exhibited at the points of disruption. It is 
for this reason that the points of conductors are so often melted. 
5. In case of a disruptive discharge through the air, the electrici¬ 
ty, even while passing through a continuous conductor well con¬ 
nected with the ground, tends in some degree to be given off in 
sparks to surrounding bodies. While in such cases, the principal 
portion of the charge has no tendency to leave the conductor, the 
quantity which tends to fly off laterally from it, is often sufficient, 
as Prof. Henry has shown, to ignite combustible materials. Light¬ 
ning-rods therefore should never be allowed to pass through material 
easily ignited. 
6. As water is much inferior as a conductor to metal, in case of 
a disruptive discharge, there will be, even when a rod is connected 
with the moist ground, considerable resistance to the passage of the 
electricity from the rod to the earth. This assistance will obvious¬ 
ly be less as the surface in contact with the moist earth is greater. 
This fact should be carefully observed in putting up lightning con¬ 
ductors. 
