lleys, erected at
intervals along the river bank, for the whole length of the town; and
power is delivered from them to shafting below the streets, and from
it into any house where it is required for manufacturing purposes.
Then we have the compressed air transmission of power, which is very
largely used for underground engines, and for the working of rock
drills in mines and tunnels.

COMPRESSED AIR LOCOMOTIVES.

We have also compressed air in a portable form, and it is now employed
with great success in driving tram-cars. I had occasion last January
to visit Nantes, where, for eighteen months, tram-cars had been driven
by compressed air, carried on the cars themselves, coupled with an
extremely ingenious arrangement for overcoming the difficulties
commonly attendant on the use of compressed air engines. This consists
in the provision of a cylindrical vessel half filled with hot water
and half with steam, at a pressure of eighty pounds on the square
inch. The compressed air, on its way from the reservoir to the engine,
passes through the water and steam, becoming thereby heated and
moistened, and in that way all the danger of forming ice in the
cylinders was prevented, and the parts were susceptible of good
lubrication. These cars, which start every ten minutes from each end,
make a journey of 33/4 miles, and have proved to be a commercial and an
engineering success. I believe, moreover, that they are capable of
very considerable improvement.

HYDRAULIC TRANSMISSION OF POWER.

Then there is, although not much used, the transmitting of power by
means of long steam pipes. There is also the transmission
hydraulically. This may be carried out in an intermittent manner, so
as to replace the reciprocating flat rods of old days; that is to say,
if two pipes containing water are laid down, and if the pressure in
those pipes at the one end be alternated, there will be produced an
alternating and a reciprocative effect at the other, to give motion to
pumps or other machinery. There is also that thoroughly well known
mode of transmission, hydraulically, for which the engineering world
owes so much to our president. We have, by Sir William Armstrong's
system, coupled with his accumulator, the means of transmitting
hydraulically the power of a central motor to any place requiring it,
and by the means of the principal accumulator, or if need be by that
aided by local accumulators, a comparatively small engine is enabled
to