May 8, 1891,] 



SCIENCE. 



255 



in regard to electricity a hundred and forty years ago, and 

 among these were the attracting power of points for an elec- 

 tric spark, and the conducting power of metals. Lightning- 

 rods were therefore introduced with the idea that the elec- 

 tricity existing in the lightning-discharge could be conveyed 

 around the building which it was proposed to protect, and 

 that the building would thus be saved. ■» 



The question as to the dissipation of the energy involved 

 was entirely ignored, naturally ; and from that time to this, 

 in spite of the best endeavors of those interested, lightning- 

 rods constructed in accordance with Franklin's principle 

 have not furnished satisfactory protection. The reason for 

 this is apparent when it is considered that this electrical en- 

 ergy existing in the atmosphere before the discharge, or, 

 more exactly, in the column of dielectric from the cloud to 

 the earth, reaches its maximum value on the surface of the 

 conductors that chance to be within the column of dielectric; 

 so that the greatest display of eaergy will be on the surface 

 of the very lightning-rods that were meant to protect, and 

 damage results, as so often proves to be the case. The very 

 existence of such a mass of metal as an old lightning-rod 

 only tends to produce a disastrous dissipation of electrical 

 energy upon its surface, — " to draw the lightning," as it is 

 so commonly put. 



Having cleared our minds, therefore, of any idea of con- 

 ducting electricity, and keeping clearly in view the fact that 

 in providing protection against lightning we must fui'uish 

 some means by which the electrical energy may be harmlessly 

 dissipated, it seems clear why it is that the use of sufficient 

 energy to dissipate a pound of copper, leaves not enough to 

 do harm to other objects around. The question naturally 

 arises how much energy there is available. There is stored 

 up in each cubic centimetre of the column of dielectric from 

 the cloud to the earth, just before the lightning-discharge, 

 an amount of electrical .energy given by the expression 



— K E^, where K is the specifle inductive capacity of the 



dielectric, and E the electromotive intensity, both in elec- 

 trostatic units. This expression is given on p. 156, Vol. I., 

 second edition, of Maxwell's "Treatise on Electricity and 

 Magnetism." Substituting the values of K and E, and re- 

 ducing, we find that the amount of energy involved amounts 

 very nearly to one foot-pound for each cubic foot of air. This 

 is, of course, a maximum value. 



When this amount of energy is reached in any cubic foot, 

 the air breaks down, and the discharge takes place, and the 

 amount of energy per cubic foot in the column of dielectric 

 reaching from the cloud to the earth cannot be uniform, but 

 must reach this maximum value along a central core, and 

 diminish gradually from this value to nothing at a consid- 

 erable distance. If we consider that the dissipation of this 

 electrical energy takes place throughout the whole length of 

 the column of dielectric from the cloud to the earth, we shall 

 see that all the energy that we have to care for in our light- 

 ning-rod is that existing in the section of the column con- 

 tained between two surfaces passing through the top and 

 foundation of our house respectively. I have said two sur- 

 faces, as doubtless they ai'e not planes: presumably they are 

 two equi-potential surfaces. 



I am now coming to a point that I want to make clear, 

 and that is, that, according to the usually accepted theories 

 of electrical action, this electrical energy is gradually stored 

 up in the column of dielectric from the cloud to the earth, 

 and that it is distributed in this column with the greatest 

 amount per cubic foot along some central core, this amount 



not exceeding one foot-pound per cubic foot, and that this 

 process can be continued until the stress is so great that the ait* 

 breaks down, when what we call a discharge of lightning: 

 takes place, and the electrical energy disappears, of course 

 only to take on some other form. You may say that the 

 electricity travels from the cloud to the earth, or from the 

 earth to the cloud, whichever you please; at any rate, there 

 is an electrical action in a vertical direction, the discharge 

 being supposed vertical. I will ask, however, whether it is 

 not true that the energy involved travels along the equi- 

 potential lines; that is, travels in the main horizontally. It 

 seems to me that it shrinks in, as it were, from the consid- 

 erable column or ellipsoid of dielectric upon the central core, 

 where it manifests itself as heat and light in the electrical 

 flash. It will, then, be clear how it is that in providing a, 

 body upon which the dissipation of energy shall take place 

 we have to guard against something not coming from above 

 or below, but coming from the side, and that this may be 

 the explanation of why it is that, so far as I have been able 

 to find, a dissipa table conductor protects the building between 

 two essentially plane surfaces passing through its upper and 

 lower ends. 



Have we not, then, in the lightning-discharge, another 

 illustration of the relation between light and electricity ? If 

 we suppose for a moment that in place of the central core 

 where the electrical energy is dissipated we were to place some 

 hot or luminous body, this body would constantly radiate 

 energy into the surrounding space, and at any instant there 

 would be in each cubic foot of this surrounding space a cer- 

 tain amount of radiant energy. Now, if this process could 

 be brought to a standstill at any moment, would not the 

 conditions be in some degree similar to those just preceding 

 the electrical discharge ? There would be need of a certain 

 force along the central core to maintain the various stresses 

 throughout the surrounding medium; and if this central 

 force were to be taken away, as it is taken away when the 

 dielectric breaks down and the spark passes, the stresses 

 could no longer be maintained, and there would be a vi- 

 bratory transmission of the energy back upon the central 

 core. 



But let all this be as it may, the main point which I would 

 urge upon your consideration is that by giving the electrical 

 energy something which experience shows it will readily 

 dissipate, that is, a conductor of varying resistance and small 

 size, we can but mitigate the effects of lightning-discharges, 

 so long as the conservation of energy holds true. I will only 

 repeat that I have so far found no case on record where the 

 dissipation of such a conductor has failed to protect the build- 

 ing under the conditions already explained. 



NOTES AND NEWS. 

 In England, says T7ie Illustrated American, the only venomous 

 snake is the viper, which frequents chalky districts, and is not to 

 be found all over the country. Perhaps these vipers are the most 

 common and vicious of the smaller snakes, seldom growing longer 

 than two feet. They abound not only in warm countries, where 

 forests are thick and men are few, but also in the coldest regions 

 of Sweden, Norway, Russia, and even Siberia, where a great 

 many exist, owing to a stupid superstition among the peasants 

 that if a viper is killed a terrible misfortune will soon befall the 

 rash slayer. The California viper builds itself a little mud hut, 

 just its own length, and probably half an inch thicker than its 

 own body. It is made of earth, fine gravel, and sometimes leaves 

 are mixed in the construction of this curious abode. It is lined 

 with a soft, silky substance, finer than cotton and silkier than 

 down. At each end there are two little doors, and when monsieur 



