PROTECTION FROM LIGHTNING. 



55 



9th. "When a house is covered by a metallic roof the latter 

 should be united in good metallic connection with the lightning 

 rods ; and in this case the perpendicular pipes conveying the 

 water from the gutters at the eaves may be made to act the part 

 of rods, by soldering strips of copper to the metal roof and pipes 

 above, and connecting them with the earth by plates of metal 

 united by similar strips of copper to their lower ends, or better to 

 the gas or water pipes of the city. In this case, however, the 

 chimneys would be unprotected, and copper lightning rods, 

 soldered to the roof and rising a few feet above the chimneys, 

 would suffice to receive the discharge." — (Prof. Joseph Henry.) 



10th. The rod should be placed on that part of the building 

 towards which showers generally direct their course, and particu- 

 larly on the chimney from which currents of heated air, serving as 

 conductors of electricity, ascend. Rods extending high above 

 buildings should be securely braced. 



11th. A lightning rod is regarded as protecting a circle whose 

 radius is double the height of the rod above the building ; hence, 

 on large buildings several rods would be required. It is safer, 

 however, to place the rods nearer one another then this rule would 

 require, especially in the case of buildings in which a large amount 

 of iron enters into the construction. There is no objection to an 

 indefinite number of rods on a building, provided a proper con- 

 - nection with the ground be established. In order to safety, it 

 should be remembered the rod must absolutely extend into water 

 or permanently moist earth. 



In the protection of vessels, copper has been advantageously 

 substituted for iron, as in the protectors planned by Sir W. Snow 

 Harris for the ships of the English Royal Navy. They consist 

 of bands of copper overlapping each other, and let into the rear 

 side of each mast. They pass down to the keel, and are continued 

 through it by means of copper bolts into the water. They con- 

 nect also with bands of copper laid under the deck beams, and 

 continued through the sides of the ship. — (American Cyclopaedia.) 



It has doubtless been noticed that in the treatment of this sub- 

 ject, theoretical considerations have been largely set aside, other- 

 wise it would have been deemed necessary to dwell upon the 

 probable and known sources of atmospheric electricity, upon the 

 manner in which clouds serve to condense electricity in the air, 

 upon the influence by induction of the positive electricity of the 

 clouds upon the earth, developing at the more elevated points 



