540 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



below their summits. Rods placed obliquely or even horizontally will 

 have the effect of discharging those parts of the clouds which in the 

 time of a thunderstorm descend to within a short distance of the ground, 

 and against which the vertical points of lightning-conductors have no 

 neutralizing action ; of course the oblique rods must have their con- 

 ductors as well as the vertical. Besides this it is advisable to place 

 all the lightning-conductors' rods of the same building in communica- 

 tion by metallic bars running along the ridges, but each should have 

 notwithstanding, as far as possible, a separate conductor. Many con- 

 ductors may without inconvenience be brought to terminate in the 

 same well, but if several bars are united into one, it must have a 

 section in proportion to the number of conductors it replaces. 



FIG. 346. Limits of protection of a system of lightning-conductors fixed on a building. 



"Of late years lightning-conductors with multiple points have been 

 much recommended as the best preservative against lateral discharges 

 of lightning. Of all the buildings that should be preserved from 

 lightning, the most important are the magazines of explosive or fulmi- 

 nating materials, such as gunpowder, gun-cap manufactories, powder 

 magazines, &c. In this case, however, it is preferable to surround 

 the place with towers of wood or masonry, on the top of which the 

 rods are fixed. The reason of this precautionary arrangement is easy 

 to understand. It is not sufficient to prevent the building from being 

 struck by lightning, the electric flow which passes off by the rods and 

 conductors must also be prevented from coming in contact with the 

 masses of air that are near the magazines where dangerous materials 

 are manufactured, or even stowed. In this air floats a fine dust of 



