No. 11. 



Observer.— No . XVIIL 



347 



positively determine the smallest number of 

 rods, or ascertain th.a greatest di^itaiicc to 

 which their protoctioii will extend under ev- 

 ery variety of circumstances. It will there- 



fore be prudent, when the buildings are very 

 large, to put up two or more rods in order to 

 keep on the safe side. 



Other considerations render this precaution 

 tiie more necessary. A highly electrified 

 cloud may pass so high above the rod (fig. 42) 

 as not to feel its influence, much less to en- 

 danger the building. But this state of secu- 

 rity may be of momentary continuance. Du- 

 ring a thunder storm the atmosphere is often 

 agitated, and the clouds tiirown into commo- 



•s! wms^ 



Fi?. 42. 

 lion by counter currents of wind. They are 

 puccessively scattered to a distance, and then 

 hurled in den.so masses. Sometimes they are 

 quickly lifted up to great heights, and then 

 as suddenly depressed near to the earth's sur- 

 face. These ari^ seasons of great danger. — 

 While the cloud (fig. '12) is passing safely 

 along without the rod roacliing to its electri- 

 fied atmosphere — a sudden convulsion of the 

 air may in an instant change its form and po- 

 sition, as represented in fig. 4.3. The cloud 

 may simultancojsiy approach near to tiie 



building — become greatly condensed, and thus 

 have the intensity of its electricity vastly in- 

 creased — and by the change in its form, the 



Fig. 43. 



greatest accumulation of the electric fluid 

 will be accumulated in the smaller extremity. 

 This combination of circumstances would be 

 productive of the greatest danger to contigu- 

 ous buildings. It is quite impossible to de- 

 termine how far the gradual operation of a 

 single point could be relied upon to avert the 

 danger in this extreme case. Prudence how- 

 ever, would seem to dictate the employment 

 of more than one rod, especially where the 

 building is very large. 

 Again, the thunder cloud A (fig. 44) may 



■%^vv/,,v.;i;ilifej§^' 



Fig. 44. 



be situated at a harmless distance, as in fig. 

 42, but a cross current of wind may suddenly 

 bring the cloud B between it and the building. 

 In this case B may form a sort of stepping- 

 stone, or intermediate point for the electricity 

 of A to strike to, and thence to the building. 

 As there would not be any appreciable inter- 



