38 



THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



The metallic bar which serves as the conductor, and has, as we have 

 seen, a section of about 15 millimetres in diameter, must be soldered 

 with care to the rod which is itself firmly fixed to the framework of 

 the ridge of the building. All the successive parts, whether vertical, 

 horizontal, or inclined must be joined by curves and soldered with the 

 same care at the points of junction. The constancy of these junc- 

 tions is further secured by branching iron supports which allow of a 

 longitudinal motion, without any lateral swaying. 



The rigid bars of the conductor are sometimes replaced by cables 

 of three or four strands of iron wire, tarred outside to prevent rust. 

 Great care must then be taken that the communication of the cable 



with the rod shall take place by as 

 large a contact as possible between the 

 metallic surface of the rod and the iron 

 wires ; these must be perfectly clean 

 and soldered to the iron of the rod, 



An essential condition, too, is that 

 all the metallic parts of the building, 

 the ridges and gutters of lead or zinc 

 the beams arid floors of iron should all 

 be in communication with each other 

 and with the lightning-conductor. 



We now come to the most essential 

 condition of all, which, if neglected, 

 would make the lightning-conductor, 

 instead of being a protection against 



the lightning, a very dangerous apparatus on the occasion of a storm. 

 It is that the conductor having once reached the ground should go 

 deep enough to be in constant communication with a water-bearing 

 stratum. For this purpose a well may have to be sunk on purpose, 

 of such a depth that in the greatest droughts the water may stand 

 in it to the height of a metre at least. Tf any water-courses, streams, 

 or rivers, of sufficient size to be never dry in times of drought, or if 

 lakes or large ponds happen to "be near the conductor, it is sufficient 

 to put it in constant communication with the water. 



Besides this, there is no reason why the conductor should not also 

 be placed in communication with the upper layer of the soil which 

 forms a supplementary reservoir when sufficiently saturated with rain ; 



FIG. 344. Junction of the vertical rod 

 to the conductor. 



