432 M. Gay-Lussac on [Due. 



venience, and the plain conical copper stem only be employed. 

 Copper does not rust to any considerable depth in the air, and 

 even if the point become somewhat blunt, the paratonnerre will 

 not thereby lose its efficacy. 



A stem of the supposed dimensions being difficult to trans- 

 port to a distance, it is cut into two parts A I, IB, fig. 2, at 

 about one-third or two-fifths of its length from the base. The 

 upper part, A D, fig. 4, fits exactly by a pyramidal tenon, D F, 

 seven or eight inches long, into the lower part, E B, and is kept 

 in its place by a pin. The stem should, however, always be 

 made of a single piece whenever that can be done.* 



Below the stem, three inches from the roof, is a cap, M N, 

 fig. 4, soldered to the body of the stem, and intended to throw off 

 the rain water which would flow down the stem, and prevent its 

 running into the interior of the building, and rotting the rafters.+ 



Immediately above the cap>, the stem is rounded for about two 

 inches to receive a split collar, with a hinge O, and two ears, 

 between which the extremity of the conductor of the paraton- 

 nerre is fixed by a bolt ; the plan of this collar is seen at P 

 below the stem. Instead of the collar, we may make a square 

 stirrup, which embraces the stem closely ; the vertical projection 

 of this is seen at Q, fig. 5, and the plan at R, fig. 6, as well as 

 the mode by which it is united to the conductor. Lastly, in 

 order to save labour, we may solder a tenon, T, fig. 7, in the 

 place of the collar ; but care must be taken not to weaken the 

 stem at this part, where it has to oppose most resistance, and 

 the collar and stirrup are preferable. The stem of the paraton- 

 nerre is fixed on the roof of building's, according to circum- 

 stances. If it is to be placed above a rafter B, figs. 7 and 8, 

 the ridge must be pierced with a hole through which the 

 foot of the stem passes, and is steadily fixed against the king- 

 post by means of several bridles, as seen in the figure. This 

 disposition is very solid, and should be preferred if local circum- 

 stances permit. 



If the stem be to be fixed on the ridge at A, fig. 8, a square 

 hole must be made through it of the same dimensions as the 

 foot of the stem; and above and below we fix, by means of bolts, 

 or two bolted stirrups, which embrace and draw the ridge toge- 

 ther, two iron plates about three-quarters of an inch thick, each 

 having a hole corresponding to that in the woodwork. The 

 stem rests by a small collet on the upper plate, against which it 

 is strongly pressed by a nut, which screws on the end of the 

 stem against the lower plate ; fig. 9 shows the plan of one of 



* The hollow part, E G, fig. 4, which receives the pyramidal tenon, D F, is made 

 thus : — A strong iron plate is rolled into a cylinder, and soldered at G to the bar B G ; 

 then by means of a mandrill of the same form as the tenon, and at successive heats, it 

 is easy to unite its edges and to give it, inside and out, the pyramidal shape. 



-|- To make the cap, an iron ring is soldered to the stem and drawn out all round on 

 the anvil, inclining the edges so as to form a very flat truncated cone. 





