furnishes steam to a two cylinder engine, which may be run at will,
either the two cylinders separately, or as a _compound_ engine. The
bronze screw is 1.3 m. in diameter, and has a pitch of 2.5 m. The vessel
has two rudders, one in front for slight speeds, and the other at the
stern. At rest, the total displacement is 52,300 kilogrammes.
This weight far exceeds what was first expected, by reason of the
superthickness given the iron plates of the vertical sides, of the
supplementary cross bracing, and of the superposition of the netting
necessary to resist the flexion of the whole. On another hand,
the tractive stress of the screw, which should reach about 4,000
kilogrammes, has never been able to exceed 1,800, because of the
numerous imperfections in the engine. It became necessary, therefore,
to steady the vessel by having her towed by the _Winkelried_, which was
chartered for such a purpose, to the General Navigation Company. It
became possible to thus carry on observations on speeds up to 27
kilometers per hour.

Fig. 3 shows how the tractive stress varies with each speed in a
theoretic case (dotted curve) in which the stress is proportional to the
square of the speed, in Madame Rothschild's boat, the _Gitana_ (curve
E), and in the Pictet high speed vessel (curve B).

The _Gitana_ was tried with speeds varying between 0 and 4 kilometers.
The corresponding tractive stresses have been reduced to the same
transverse section as in the Pictet model in order to render the
observations comparable. At slight speeds, and up to 19.5 kilometers per
hour, the _Gitana_, which is the sharper, runs easier and requires a
slighter tractive stress. At such a speed there is an equality; but,
beyond this, the Pictet boat presents the greater advantages, and, at a
speed of 27 kilometers, requires a stress about half less than does the