locomotives with four coupled wheels, diameters 6 ft. to 7 ft. There
is therefore an important difference between the diameters of the
coupled wheels of 7 ft. and those of 8 ft. 3 in., as conceived by M.
Estrade. However, the transition is not illogically sudden, and if the
conception is a bold one, "it cannot," says M. Nansouty, "on the other
hand, be qualified as rash."

He goes on to consider, in the first place: Especial types of
uncoupled wheels, the diameters of which form useful samples for our
present case. The engines of the Bristol and Exeter line are express
tender engines, adopted on the English lines in 1853, some specimens
of which are still in use.[1] These engines have ten wheels, the
single drivers in the center, 9 ft. in diameter, and a four-wheeled
bogie at each end. The driving wheels have no flanges. The bogie
wheels are 4 ft. in diameter. The cylinders have a diameter of 16½ in.
and a piston stroke of 24 in. The boiler contains 180 tubes, and the
total weight of the engine is 42 tons. These locomotives, constructed
for 7 ft. gauge, have attained a speed of seventy-seven miles per
hour.

[Footnote 1: M. Nansouty is mistaken. None of the Bristol and
Exeter tank engines with. 9 ft. wheels are in use, so far as we
know. ED. E.]

The single driver locomotives of the Great Northern are powerful
engines in current use in England. The driving wheels carry 17 tons,
the heating surface is 1,160 square feet, the diameters of the
cylinders 18 in., and that of the driving wheels 8 ft. 1 in. We have
here, then, a diameter very near to that adopted by M. Estrade, and
which, together with the previous example, forms a precedent of great