other mountain roads in
Europe. In the Wetli system, instead of this rail and the pinion on
the vehicle engaging it, there is a drum having a helicoidal thread
which engages with triangular rails. This drum is attached to the
locomotive. The construction will be readily understood from the
illustrations given herewith, which we take from _La Nature_. The
thread on the drum is precisely that which would be formed could a
rail similar to one of the central angular rails be wrapped around it;
so that it always is in contact with the mid rails, and necessarily
prevents any bodily sliding or rolling of the vehicles over the
regular track when the drum is held motionless. The V-shaped mid rails
are securely fastened to horizontal iron ties, which rest on wooden
traverses. The angle of the V is 50 deg.; the distance between any two
traverses is 2.8 feet.

[Illustration: Fig. 1.--THE WETLI MOUNTAIN RAILROAD.]

The locomotive has three coupled axles, on the mid one of which the
drum is attached so as to be raised or lowered to engage the rails at
the will of the engineer: it being possible to cause it to act on the
rails with a pressure of 3.7 tons. The diameter of the drum is 2.14
feet. Its spiral thread is of steel, very solidly attached, and so
made as to grip the rails to a distance of 0.6 inch below the level of
the track. In order to insure this contact, on the drum axle are two
pulleys which run on the exterior road, and of which the diameter
determines the depth of the hold of the threads. These pulleys are
34.7 inches in diameter, while the driving wheels are very slightly in
excess, to provide for the use of tyres.

M. Wetli's invention, as we have described it, was placed between
Woedensweil and Einsiedlen, Switzerland. The difference in altitude
between these points is 1,513 feet, the distance 9.6 miles. The grade
is from 4 to 5 per cent over the first six miles of the way, and
subsequently decreases to 1 per cent. The Wetli railroad was
established last October only on the heavy grade, that is, the first
six miles.

[Illustration: Fig. 2.]

Early in November, trial trips were made which did not prove
satisfactory. Sometimes the drum thread gripped the triangular rails
properly and acted well; again it would wedge itself upon them, and
often would simply roll over their tops without engaging at all. After
the first trials, during which very many of the rails were broken, M.
Welti re-adjusted the drum thread.