de--one for the upper and one for the lower run. Thus, each car traveled
only half the total distance. The two cars were connected, as in the
Backmann system, by steel cables running over sheaves at the top,
balancing each other and eliminating the need for counterweights. Two
driving rams were used. By being placed beneath the upper car, their
cylinders extended downward only the 262 feet to the second platform and
so did not project beyond the confines of the system itself.[15] In making
the upward or downward trip, the passengers had to change from one car to
the other at the intermediate platform, where the two met and parted (fig.
39). This transfer was the only undesirable feature of what was, on the
whole, a thoroughly efficient and well designed work of elevator
engineering.

[Illustration: Figure 32.--The Otis distributor, with valves shown in
motionless, neutral position. Since the main valve at all times was
subjected to the full operating pressure, it was necessary to drive this
valve with a servo piston. The control cable operated only the servo
piston's valve. (Adapted from Gustave Eiffel, _La Tour de Trois Cents
Metres_, Paris, 1900, p. 130.)]

[Illustration: Figure 33.--General arrangement of the Roux Combaluzier and
Lepape elevator.]

[Illustration: Figure 34.--Roux, Combaluzier and Lepape machinery and
cabin at the Tower's base. (From _La Nature_, Aug. 10, 1889, vol. 17, p.
168.)]

In operation, water was admitted to the two cylinders from a tank on the
third platform. The resultant hydraulic head was sufficient to force out
the rams and raise the upper car. As the rams and car rose, the rising
water level in the cylinders caused a progressive reduction of the
available head. This negative effect was further heightened by the fact
that, as the rams moved upward, less and less of their length was
buoyed by the water within the cylinders, increasing their effective
weight. These two factors were, however, exactly compensated for by the
lengthening of the cables on the other side of the pulleys as the lower
car descended. Perfect balance of the system's dead load for any position
of the cabins was, therefore, a quality inherent in its design. However,
there were two extreme conditions of live loading which required
consideration: the lower car full and the upper empty, or vice versa. To
permit the upper car to descend under the first condition, the plungers
were made sufficiently heavy, by the add