period, it is claimed, is now passed, and in the new installation it
is possible to put into practice all the valuable lessons learned at
St. Fargeau, to say nothing of the more favorable natural conditions
under which the extension is being started and the improvements in the
compression of the air made by Mr. Popp and Professor Riedler, and to
which we shall refer later.

Chiefly in consequence of the high value of the ground, vertical
engines were adopted at the new station; the proximity to the river
made the foundations somewhat costly, and the risk of occasional
floods rendered it desirable to set the level of the engine bedplates
20 inches above the floor of the building; the foundations of the
engines are continuous, but are quite independent of the building.
There are three compressing cylinders in each set of engines, one
being above each steam cylinder. Two of these are employed to compress
the air to about 30 lb. per square inch, after which it passes into a
receiver and is cooled; it is then admitted into the third or final
compressing cylinder and raised to the working pressure at which it
flows into the mains. In the illustrations, h, m, and b are the high,
intermediate, and low pressure cylinders of one set of engines; as
will be seen, each cylinder is on a separate frame connected by
girders; directly above the cylinders are the two low and the one high
pressure air cylinders, b¹, m¹, and h¹ respectively. The former
deliver the air compressed to the first stage into the receiver, T¹
(see Fig. 5), whence it passes into the third compression cylinder,
and thence by a main into the cylinders, R R, which are in direct
communication with the delivery mains; these mains terminate in the
subway, T. The water for condensation is brought into the engine house
by the channel, C, and the condenser pumps, a, draw direct from this
supply; the discharge main back to the river is shown at A. The