NATURAL PHILOSOPHT. 105 



connected by a fine wire with the collecting conductor. The height of the 

 match was three feet above the ground, and the observer at the electrometer 

 lay on the ground to render the electrical influence of his own body on the 

 match insensible. The result showed a difference of potentials between the 

 earth (negative) and the air (positive) at the match equal to that of one 

 hundred and fifteen elements of Daniel's batteiy, and, therefore, at that time 

 and place, the aerial electro-motive force per foot amounted to that of thirty- 

 eight Daniel's cells. 



ON THE THEORY AND CONSTRUCTION OF LIGHTNING-RODS. 



The theory of a thunder-cloud and a conductor ought to be better under- 

 stood in this country than it is, seeing that it lies almost in a nutshell. 

 Lightning obeys one unvarying law, it uniformly follows the best continuous 

 conductor; but no conductor can be considered a good one unless it is con- 

 tinuous. Numerous evidences of this have been afforded by broken or other- 

 wise defective rods. A flash takes the rod and follows it to where the break 

 exists, then finds its next best conductor within the building, immediately 

 opposite the spot where it discovered the break, crashes through the wall, 

 perhaps where the family are sitting, and deals death around it, finding its 

 way into the earth by tortuous channels, the stove-pipe, the gas-pipe, or, in 

 their absence, by shattering the wood-work and plastering. Defective rods 

 of any kind are mere traps to bring lightning into a house, instead of keep- 

 ing it out. They are the most dangerous fixture a man can have about him ; 

 and though numerous crudely written paragraphs are constantly afloat of 

 houses being damaged, though provided with rods, yet it may be assumed 

 as absolutely certain that in every such instance the rod has been miserably 

 out of order, or put up meanly and cheaply by direction of a penurious 

 owner, or by an ignorant and incompetent peddler. The principle of protec- 

 tion developed by Franklin remains sound ; and all that is needed to secure 

 perfect immunity from danger is a strict adherence to what we know it 

 demands as the condition of safety. When the usual term for thunder-storms 

 is coming on, every careful householder should have his lightning rods ex- 

 amined, and, if found defective, put in perfect order. The joints should be 

 seen to be close and tight, for continuity is indispensable to safety. If the 

 winter's storm has bent that part which projects above the roof, it should be 

 taken down and straightened. See, also, that the lower section which goes 

 into the ground has not rusted off, as is often the case. And this thorough 

 examination should be made every year. 



Thunder-clouds are charged with different degrees of intensity, some 

 heavily, some lightly. Some sweep over the earth at a greater altitude 

 than others. Those which hang low discharge their contents, whether of 

 water or electricity, with the greatest energy. All our thunder-storms, with 

 few exceptions, come up from the northwest. Hence the conductors should 

 be erected at those points of the building with which the cloud will first 

 come in contact. This is necessary, because every thunder-cloud is sur- 

 rounded by an electric atmosphere, which precedes the cloud itself. This may 

 be easily verified by placing the knuckle to the conductor as the cloud ap- 

 proaches. Sparks will frequently be drawn from it while the thunder yet 

 rolls in the distance, showing that the electrical haze has already enveloped 

 the building, and that the rod is silently conducting the fluid into the earth. 

 The rod is already performing its functions with the mere electrical atmos- 



