ELECTRICAL APPARATUS. 309 



electricity, nor is this supposition in any way weakened by 

 our experimental researches. If we compare the properties 

 of electricity with those of lightning, we shall find them 

 closely analogous, or rather identical. To Franklin, whose 

 active mind was constantly directed to practical application 

 of the facts disclosed by science, we are indebted for the 

 suggestion of a method of partially defending buildings from 

 the dreaded effects of lightning. His method was, to erect 

 by the side of the building to be protected, a continuous 

 metallic rod, in perfect communication with the earth ; and 

 experience has fully demonstrated the value of this pre- 

 caution. 



The conductor should penetrate the ground sufficiently 

 deep to be in close contact with a stratum of moist soil, and 

 be carried above the highest point of the building. Great 

 care should be taken that every part of the rod be perfectly 

 continuous, and that its substance be sufficient to prevent 

 any chance of its being melted ; perfect security on this 

 head is arrived at by having a rod three quarters of an inch 

 thick. It has been proved, that conductors erected with 

 these precautions will protect a circular space of a radius 

 double the height above the highest point of the building to 

 which they are attached. 



The little arrangement Fig. 359 



(Fig. 369) amusingly illus- 

 trates the use of a continu- 

 ous conductor. A board, 

 about three quarters of an 

 inch thick, and shaped like 

 the gable end of a house, is 

 fixed perpendicularly upon 

 another board, upon which 

 a glass pillar also is fixed in 

 a hole about eight inches dis- 

 tant from the gable-shaped 

 board. A small hole, about 

 a quarter of an inch deep, 

 and nearly an inch wide, is 

 cut in the gable-shaped board, and this is filled with a square 

 piece of wood of nearly the same dimensions. It should be 

 nearly of the same dimensions, because it must go so easily 

 into the hole, that it may drop off by the least shaking of 

 the instrument. A brass wire is fastened diagonally to this 



