646 Mr. Nikola Tesla [Feb. 4, 



considerable. Since the density must be unevenly distributed, either 

 in consequence of the irregularity of the earth's surface, or on account 

 of the condition of the atmosphere in various places, the effect pro- 

 duced would accordingly vary from place to place. Considerable 

 variations in the temperature and pressure of the atmosphere may in 

 this manner be caused at any point of the surface of the earth. The 

 variations may be gradual or very sudden, according to the nature of 

 the original disturbance, and may produce rain and storms, or locally 

 modify the weather in any way. 



From many experiences gathered in the course of these investiga- 

 tions it appears certain that in lightning discharges the air is an 

 element of importance. For instance, during a storm a stream may 

 form on a nail or pointed projection of a building. If lightning 

 strikes somewhere in the neighbourhood, the harmless static dis- 

 charge may, in consequence of the oscillations set up, assume the 

 character of a high-frequency streamer, and the nail or projection 

 may be brought to a high temperature by the violent impact of the 

 air molecules. Thus, it is thought, a building may be set on fire 

 without the lightning striking it. 



In like manner small metallic objects may be fused and volati- 

 lised — as frequently occurs in lightning discharges — merely because 

 they are surrounded by air. Were they immersed in a practically 

 continuous medium, such as oil, they would probably be safe, as the 

 energy would have to spend itself elsewhere. 



An instructive experience having a bearing on this subject is the 

 following : — A glass tube of an inch or so in diameter and several 

 inches long is taken, and a platinum wire sealed into it, the wire 

 running through the centre of the tube from end to end. The tube 

 is exhausted to a moderate degree. If a steady current is passed 

 through the wire it is heated uniformly in all parts and the gas in 

 the tube is of no consequence. But if high frequency discharges are 

 directed through the wire, it is heated more on the ends than in the 

 middle portion, and if the frequency, or rate of charge, is high 

 enough, the wire might as well be cut in the middle as not, for most 

 of the heating on the ends is due to the rarefied gas. Here the gas 

 might only act as a conductor of no impedance, diverting the current 

 from the wire as the impedance of the latter is enormously increased, 

 and merely heating the ends of the wire by reason of their resistance 

 to the passage of the discharge. But it is not at all necessary that 

 the gas in the tube should be conducting ; it might be at an 

 extremely low pressure, still the ends of the wire would be heated, 

 as, however, is ascertained by experience, only the two ends would 

 in such case not be electrically connected through the gaseous 

 medium. Now what with these frequencies and potentials occurs in 

 an exhausted tube, occurs in the lightning discharge at ordinary 

 pressure. 



From the facility with which any amount of energy may be carried 

 off through a gas, it is concluded that the best way to render harm- 



