440 FKAaMENTS OF SCIENCE. 



nets of a second and larger machine. Wilde's prin- 

 ciple, it may be added, is also applied on the Thames 

 Embankment and the Holborn Viaduct; a small 

 Gramme machine being used in each case to excite the 

 electro-magnets of the large one. 



The Farmer- Wallace machine is also an apparatus 

 of great power. It consists of a combination of bobbins 

 for induced currents, and of inducing electro-magnets, 

 the latter being excited by the method discovered by 

 Siemens and Wheatstone. In the machines intended 

 for the production of the electric light, the electro- 

 motive force is so great as to permit of the introduction 

 of several lights in the same circuit. A peculiarly 

 novel feature of the Farmer- Wallace system is the shape 

 of the carbons. Instead of rods, two large plates of 

 carbons with bevelled edges are employed, one above 

 the other. The electric discharge passes from edge to 

 edge, and shifts its position according as the carbon is 

 dissipated. The duration of the light in this case far 

 exceeds that obtainable with rods. I have myself seen 

 four of these lights in the same circuit in Mr. Ladd's 

 workshop in the City, and they are now, I believe, 

 employed at the Liverpool Street Station of the Metro- 

 politan Railway. The Farmer- Wallace * quantity 

 machine' pours forth a flood of electricity of low 

 tension. It is unable to cross the interval necessary for 

 the production of the electric light, but it can fuse 

 thick copper wires. When sent through a short bar of 

 iridium, this refractory metal emits a light of extra- 

 ordinary splendour. 1 



The machine of M. de Merit ens, which he has 

 generously brought over from Paris for our instruction, 

 is the newest of all. In its construction he falls back 



1 The iridium light was shown by Mr. Ladd. It brilliantly illu- 

 minated the theatre of the Koyal Institution. 



