The wires have no ground-connection outside of the cave.
They go out from the positive brush of the dynamo; there is a ground-connection through the negative brush; the other ends of the wire return to the cave, and each is grounded independently." "No, no, that won't do!" "Why ?" "It's too expensive--uses up force for nothing.

You don't want any ground-connection except the one through the negative brush.
The other end of every wire must be brought back into the cave and fastened independently, and _without_ any ground-connection.
Now, then, observe the economy of it.

A cavalry charge hurls itself against the fence; you are using no power, you are spending no money, for there is only one ground-connection till those horses come against the wire; the moment they touch it they form a connection with the negative brush _through the ground_, and drop dead.

Don't you see ?--you are using no energy until it is needed; your lightning is there, and ready, like the load in a gun; but it isn't costing you a cent till you touch it off.

Oh, yes, the single ground-connection--" "Of course! I don't know how I overlooked that.