with diameters never before tried, but these six coupled wheels
constitute the whole rolling length, while in the above engines a
leading axle or a bogie must be taken into account, independent, it is
true, but which must not be lost sight of, and which will in a great
measure equalize the difficulties of passing over the curves.

"Is it opposed to absolute security to attack the line with driving
wheels? This generally admitted principle appears to rest rather on
theoretic considerations than on the results of actual experience. M.
Estrade, besides, sets in opposition to the disadvantages of attacking
the rails with driving wheels those which ensue from the use of wheels
of small diameter as liable to more wear and tear. We should further
note with particular care that the leading axle of this locomotive has
a certain transverse play, also that it is a driving axle. This
disposition is judicious and in accordance with the best known
principles."

A careful perusal of M. Nansouty's memoir leaves us in much doubt as
to what M. Estrade's views are based on. So far as we understand him,
he seems to have worked on the theory that by the use of very large
wheels the rolling resistance of a train can be greatly diminished. On
this point, however, there is not a scrap of evidence derived from
railway practice to prove that any great advantage can be gained by
augmenting the diameters of wheels. In the next place, he is afraid
that he will not have adhesion enough to work up all his boiler power,
and, consequently, he couples his wheels, thereby greatly augmenting
the resistance of the engine. He forgets that large coupled wheels
were tried years ago on the Great Western Railway, and did not answer.
A single pair of drivers 8 ft. 3 in. in diameter would suffice to work
up all the power M. Estrade's boiler could supply at sixty miles an