600 TRAN3ACTtONS OF THE AMERICAN INSTITUTE. 



volatilizes, and -when brought again to the solid state, it is found 

 to he quite free from impurities. The residue is a hard solid, for 

 "which uses have already been found. 



EFFECTS OF LIGHTNING. 



Emile Duchemin, lately communicated to the French Academy 

 important observations on a thunderbolt which struck the light- 

 house at Fecamp. The gigantic electric spark passed through the 

 "tower, breaking everything in its course, even as far as the marble 

 pavement, which it smashed before entering the earth. The tower 

 was provided with a lightning conductor, but, on examination, it 

 was found that it was carried down into a small cistern hollowed 

 out of the chalk, and lined with Portland cement. It may be useful 

 here to repeat the conditions necessary for safety. 



1. The points of the conductors must be sharp. 



2. The conductor must be of proper proportions, and without a 

 break in its entire length. 



3. When not terminated in a well, the conductor must be carried 

 down to a point where the ground is constantly moist. 



GREAT STONE ARCH. 



The bridge used for carrying the Washington aqueduct across 

 Cabin John creek, is a stone arch of greater span than any other 

 in existence, being 220 feet, or twenty feet longer than that of the 

 Grosveuor bridge at Chester, England. The intrados of the arch 

 is struck with a radius of 134.2852 feet, while the radius of the 

 extrados is 143.2695 feet. The depth of the voussoirs, or arch- 

 stones, is six feet two inches at the springings, and four feet two 

 inches at the crown. Outside the voussoirs is another series of 

 arch-stones, which make the total thickness of the arch at the 

 springings twenty feet four inches. The rise of the arch is 57.2624 

 feet. The abutments are formed of the solid rock on each side of 

 the creek. This rock is stepped down, and the steps are filled 

 • with concrete, on which are the footings of the arch-bed. The 

 r haunches and abutments are lightened by relieving arches — four 

 f-on the eastern side of the creek, and five on the western, extending 

 through half the thickness of the bridge. In mid- winter the key- 

 stone was inserted, and it was noticed afterward that a rise of 

 'temperature lifted the arch from the centering, which was not' 

 'removed until nearly a year later. The width of the bridge on 

 ' the face of the arch is twenty feet four inches. The circular 

 . water-channel is nine feet in diameter. The bridge was designed 



