e instruments above.--Allow me to
introduce you to the Battery Inspector."

The man in grey bowed to the Inspector, who was a tall, powerful man,
quite fit, apparently, to take charge of a battery of horse artillery if
need were.

"A singular place," remarked the detective, looking sharply round the
large room, whose dimensions were partially concealed, however, by the
rows of shelving which completely filled it from floor to ceiling.

"Somewhat curious," assented the Inspector; "you see our batteries
require a good deal of shelving. All put together, there is in this
room about three miles of shelving, completely filled, as you see, with
about 22,000 cells or jars. The electricity is generated in these jars.
They contain carbon and zinc plates in a solution of bichromate of
potash and sulphuric acid and water. We fill them up once every two
weeks, and renew the plates occasionally. There is a deal of sulphate
of copper used up here, sir, in creating electricity--about six tons in
the year. Pure copper accumulates on the plates in the operation, but
the zinc wears away."

The detective expressed real astonishment and interest in all this, and
much more that the Inspector told him.

"Poisonous stuff in your jars, I should fancy?" he inquired.

"Rather," replied the Inspector.

"Does your door ever stand open?" asked the detective.

"Sometimes," said the other, with a look of slight surprise.

"You never received a visit down here from a mad dog, did you?" asked
the man in grey.

"Never!"

"I only ask the question," continued the other, in a careless tone,
"because I once read in the newspapers of a poodle being chased into the
Post-Office and never heard of again. It occurred to me that poison
might account for it.--A curious-looking thing here; what is it?"

He had come to a part of the Battery Room where there was a large frame
or case of dark wood, the surface of which was covered with innumerable
brass knobs or buttons, which were coupled together by wires.

"That is our Battery Test-Box," explained the Inspector. "There are
four thousand wires connected with it--two thousand going to the
instruments up-stairs, and two thousand connected with the battery-jars.
When I complete the circuit by connecting any couple of these buttons,
the influence of the current is at once perceived."

He took a piece of charcoal, as he spoke, and brought it into contact
with two of the knobs. The result was to c