Mechanical Science, 145 



to each other, and is then screwed into the iron stem, and secured 

 there also by a steady pin. 



The conductor should be about three-quarters of an inch square, 

 and as before stated, should reach from the foot of the stem to the 

 ground. It should be firmly united to the stem, by being tightly 

 jammed between the two ears of a collar, by means of a bolt. 

 The conductor should be supported parallel to the roof, at about 

 six inches distance from it, by forked stanchions, and after turn- 

 ing over the cornice of the building, without touching it, should 

 be brought do\\Ti the wall, and to which it should be fastened by 

 means of cramps. At the bottom of the wall, it is bent at right 

 angles, and carried in that direction 12 or 15 feet, when it turns 

 down into a well. 



Since iron buried in the ground in immediate contact with moist 

 earth soon becomes covered with rust, and is by degrees destroyed, 

 the conductor should be placed in a trough filled with charcoal, in 

 tlie following manner. Having made a trench in the earth, about 

 two feet deep, a row of bricks is laid on their broad faces, and on 

 them others on edge ; a stratum of bakers' ashes {braise de bou- 

 langer) is then strewed over the bottom bricks, about two inches 

 thick, on which the conductor is laid, and the trough then filled 

 up with more ashes, and closed by a row of bricks laid along the 

 top. Iron thus buried in charcoal, will undergo no change in 30 

 years. After leaving the trough, the conductor passes through 

 the side of the well before alluded to, and descends into the water 

 to the depth of at least two feet, below the lowest water line. 

 The extremity of the conductor usually terminates in two or three 

 branches, to give a readier passage to the lightning into the water. 

 If there be no well at liand, a hole must be made in the ground, 

 with a six-inch auger, to the depth of about 10 or 15 feet, and the 

 conductor passed to the bottom of it, placing it carefully in the 

 centre of the hole, which is then to be filled up ^vith bakers* 

 ashes, rammed down as hard as possible, all round the conductor. 

 In a dry soil, or on a rock, the trench to receive the conductor 

 should be at least twice as long as that for a common soil, and 

 even longer, if thereby it be possible to reach moist ground. 

 Should the situation not admit of the trencli being much increased 

 in length, others, in a transverse direction, should be made, in 

 which small bars of iron, surrounded by ashes are placed, and • 

 connected with the conductor. In general, the trench should be 

 made in the dampest, and consequently lowest spot near the build- 

 ing, and the water gutters made to discharge their waters over it, 

 so as to keep it always moist. Too great precautions cannot be taken 

 to give the lightning a ready passage to the grovnd^for it is chiefly 

 on this^ that the efficacy of a paratonnerre depends. 



As iron bars are difficult to bend according to the projections 

 of a building, it has been proposed to substitute metallic ropes in 



Vol. XIX. L 



