heir minds, and Robin gradually began
to understand a little more of the nature of that subtle fluid--if we
may venture so to call it--under the influence of which he had been
born.

"Come, Madge," he said one day, throwing on his cap, "let us go and play
at cables."

Madge, ever ready to play at anything, put on her sun-bonnet and
followed her ambitious leader.

"Is it to be land-telegraphs to-day, or submarine cables?" inquired
Madge, with as much gravity and earnestness as if the world's welfare
depended on the decision.

"Cables, of course," answered Robin, "why, Madge, I have done with
land-telegraphs now. There's nothing more to learn about them. Cousin
Sam has put me up to everything, you know. Besides, there's no mystery
about land-lines. Why, you've only got to stick up a lot o' posts with
insulators screwed to 'em, fix wires to the insulators, clap on an
electric battery and a telegraph instrument, and fire away."

"Robin, what _are_ insulators?" asked Madge, with a puzzled look.

"Madge," replied Robin, with a self-satisfied expression on his pert
face, "this is the three-hundred-thousandth time I have explained that
to you."

"Explain it the three-hundred-thousand-and-first time, then, dear Robin,
and perhaps I'll take it in."

"Well," began Robin, with a hypocritical sigh of despair, "you must know
that everything in nature is more or less a conductor of electricity,
but some things conduct it so well--such as copper and iron--that they
are called _conductors_, and some things--such as glass and
earthenware--conduct it so _very_ badly that they scarcely conduct it at
all, and are called _non-conductors_. D'ee see?"

"Oh yes, I see, Robin; so does a bat, but he doesn't see well. However,
go on."

"Well, if I were to run my wire through the posts that support it, my
electricity would escape down these posts into the earth, especially if
the posts were wet with rain, for water is a good conductor, and Mister
Electricity has an irresistible desire to bolt into the earth, like a
mole."

"Naughty fellow!" murmured Madge.

"But," continued Robin impressively, "if I fix little lumps of glass
with a hole in them to the posts, and fix my wires to these, Electricity
cannot bolt, because the glass lumps are non-conductors, and won't let
him pass."

"How good of them!" said Madge.

"Yes, isn't it? So, you see," continued Robin, "the glass lumps are
insulators, for they cut the electricity off