418 



EECENT PROGRESS IN PHYSICS. 



1 



The charge is 



After all dusfc and moisture are properly removed from it, one half 

 of the intended charge is put into the hole, the cartridge is insert^'^ 



and the remaining gunpowder 

 F'gss- filled in above it. Thus, the ^'s-^- 



cartridge is in the middle of the 



charge, and the long conducting 



wires still project several feet 



above the rock 



not tamped in the usual way 

 A wadding of straw or tow is 



carefully pushed down the hole, 



so that a space filled with air, of 



variable size according to circum- ^<^^ 



stances, remains between it and ''^^ 



the charge. Upon this wad dry #^^ 



sand is poured until the bore is :^; 



entirely filled, ^■ 



The two separate ends of the cartridge wires 

 must now be brought into connexion by con- 

 ducting wires with the battery 60 to 90 feet 

 ofi". The conductors are also covered wires 

 about one line in diameter, placed side by side 

 and kept close together by being wound over 

 with twine throughout their whole length, 

 with the exception of their extremities^ where they are to be connected 

 with the battery and with the cartridge wires. 



It is sometimes necessary that the person who has to ignite the 

 cbarge should still be further off from the charge than the battery is, 

 and for this purpose an arrangement must be made by which the 

 circuit may be closed from a distance. 



Eoberts contrived the following arrangement for this purpose : upon 

 two opposite ends of the box which contains the battery two v/ooden 

 posts are erected, connected above by a wooden rod of one inch in 

 diameter. At one end a tin disk, A, fig. 60, three to four inches in 



Fi". 80. 



% 



^ 



^■«!v\^\ 



diameter, is fastened, to which a wire is soldered conducting to one, 

 say the positive pole of the battery. Another tin disk, B, is fastened 

 to a tin tube, made to slide easily on the rod, and this is kept from 

 A by a spiral spring. One end, D, of this spiral is connected with 

 one of the conducting wires, while the other conducting wire leads to 

 the negative pole of the battery. The disk B is therefore connected 



