EXPERIMENT STATION BULLETINS. 



195 



1907, expressed in parts in one hundred. 



The rush in any case, however, is likely to be rather violent, and, 

 like an avalanche, it will not take the easiest path provided for it, as 

 if it were a trickling stream, but will crash through obstacles and 

 make its own path, some portions of it taking paths which would 

 be quite unexpected. Hence, no one path can be said to protect others, 

 and the only way to protect a building with absolute completeness is 

 to inclose it wholly in metal. An invisible cage or framework of iron 

 wires, however, descending vertically down its salient features, with the 

 utilization of any metal in its construction, suffices for all practical 

 purposes, unless the building is a powder magazine. 



The effect of points, and of rain also, in gradually dissipating a charge, 

 and thereby contributing, to safety, has long been understood; but 

 the feature which has not been known is that there are cases where 

 points are wholly inoperative, viz, when the energy is stored between 

 cloud and cloud, instead of between cloud and earth, and when the 

 initial discharge takes place from one cloud to another; then the lower 

 cloud is liable suddenl}- to overflow, to earth through a region in which 

 there was no previous preparation, and where any number of points, or a 

 rain shower, or any other form of gentle leak, would have been quite 

 inoperative; Then can a violent discharge occur to even the sharpest 

 point ; and a hot column of air, such as rises up a chimney, is even pre- 

 ferred to a conductor. These are the flashes against which points and 

 rain are no protection, and these are probably those which do the most 

 damage to protected buildings. But it must be understood that when 

 a flash does occur through a building, it matters little which kind of 

 a flash it is — both can be equally sudden and violent — but if the build- 

 ing is well provided with points, the first or prepared kind is not likely 

 to occur, save in exceptional cases, the dangerous liability is then the 

 sudden or overflow variety of flash. 



These, then, are the two points of novelty: 



1. The possible occurrence of a totally unprepared-for and sudden 

 flash in previously unstrained air, by reason of overflow from a dis- 

 charge initiated elsewhere, what is called the B spark, occurring as the 

 secondary result of an A spark. 



2. The effect of electrical inertia or momentum, so that the discharge 

 is not a simple leak or flow in one direction, but a violent oscillation 

 and splash or impulsive rush, much more like an explosion, and occur- 

 ring in all directions at once, without much regard to the path which 



