efficient use on both level surfaces and heavy grades.

[Illustration: COTTRAU'S LOCOMOTIVE FOR ASCENDING STEEP GRADES.]

As well known, all the engines employed on level roads are provided with
large driving wheels, which, although they have a comparatively feeble
tractive power, afford a high speed, while, on the contrary, those that
are used for ascending heavy grades have small wheels that move slowly,
but possess, as an offset, a tractive power that enables them to
overcome the resistances of gravity.

M. Cottrau's engine possesses the qualities of both these types, since
it is provided with wheels of large and small diameter, that may be used
at will. These two sets of wheels, as may be seen from the figure, are
arranged on the same driving axle. The large wheels are held apart
the width of the ordinary track, while the small wheels are placed
internally, or as in the case represented in the figure, externally.
These two sets of wheels, being fixed solidly to the same axle, revolve
together.

On level surfaces the engine rests on the large wheels, which revolve
in contact with the rails of the ordinary track, and it then runs with
great speed, while the auxiliary wheels revolve to no purpose. On
reaching an ascent, on the contrary, the engine meets with an elevated
track external or internal to the ordinary one, and which engages with
the auxiliary wheels. The large wheels are then lifted off the ordinary
track and revolve to no purpose. In both cases, the engine is placed
under conditions as advantageous as are those that are built especially
for the two types of roads. The idea appears to be a very ingenious one,
and can certainly be carried out without disturbing the working of the