688 PR A GMENTS 
may have an indefinite number of islands, producing an 
indefinite subdivision of the trunk stream, so in the case of 
ele.-.tricity we may have instead of two branches, any number 
of branches, the current dividing itself among them, in 
accordance with the law which fixes the relation of flow to 
resistance. 
Let us apply this knowledge. Suppose an insulated 
copper rod, which we may call an "electric main," to be 
laid down along one of our streets, say along the Strand. 
Let this rod be connected with one end of a powerful vol- 
taic battery, a good metallic connection being established 
between the other end of the battery and the water-pipes 
under the street. As long as the electric main continues 
unconnected with the water-pipes, the circuit is incomplete 
and no current will flow; but if any part of the main, 
however distant from the battery, be connected with the 
adjacent water-pipes, the circuit will be completed and 
the current will flow. Supposing our battery to be at 
Charing Cross, and our rod of copper to be tapped oppo- 
site Somerset House, a wire can be carried from the j-od 
into the building, and the current passing through the 
wire may be subdivided into any number of subordinate 
branches, which reunite afterward and return through the 
water-pipes to the battery. The branch currents may be 
employed to raise to vivid incandescence a refractory metal 
like iridium or one of its alloys. Instead of being tapped at 
one point, our main may be tapped at one hundred points. 
The current will divide in strict accordance with law, its 
power to produce light being solely limited by its strength. 
The process of division closely resembles the circulation of 
the blood; the electric main carrying the outgoing current 
representing a great artery, the water-pipes carrying the 
return current representing a great vein, while the inter- 
mediate branches represent the various vessels by which 
the blood is distributed through the system. This, if I 
understand aright, is Mr. Edison's proposed mode of illu- 
mination. The electric force is at hand. Metals suffici- 
ently refractory to bear being raised to vivid incandescence 
are also at hand. The principles which regulate the divi- 
sion of the current and the development of its light and 
heat are perfectly well known. There is no room for a 
"discovery," in the scientific sense of the term, but there 
is ample room for the exercise of that mechanical ingenuity 
