"Hills and loose, slippery ground are well known to be the _res
adversæ_ of steam-carriages; on ordinary level roads they roll
along with rapid facility. In every ascent there are two additional
circumstances inimical to progressive motion. One is, that carriages
press less on the ground of a hill than on that of a plain, thus
giving the wheels a less forcible grasp or bite. But this may be
easily remedied in the structure of a carriage, and is not of very
material consequence in the steepest hills that we have. The other is
more serious. When a carriage ascends a hill, the weight or gravity of
the whole is decomposable into two--one perpendicular, and the other
parallel to the road. The former constitutes the pressure on the road,
the latter the additional work the engine has to perform. Universally
this is the same part of the whole carriage and its load together,
which the perpendicular ascent of the hill is of its length. With
these principles, if we knew the bite of the wheels on the road,
we could at once subject the powers of Mr. Gurney's carriage to
calculation.

"Now, from one of the experiments made in the barrack-yard, at
Hounslow, I find we can approximate towards it. For instance, with one
wheel only fixed to the 'carriers,' the carriage drew itself and load
of water and coke (about 1 ton), with three men on it, and a wagon
behind of 16 cwt. containing 27 soldiers. This, at the rate of 1-1/2
cwt. to a man, in round numbers is 4 tons. Estimating the force of
traction of spring carriages at a twelfth of the total weight, it
consequently gives a hold or bite on the road of 1-12 of 4 tons, or
6 2-3rds cwt. per wheel, or 13 1-3rd cwt. for the two wheels. This is
likewise the propelling force of the carriage. Supposing, therefore,
we were ascending a hill of 1 foot rise in 8, which I am assured
exceeds in steepness any hill we have, we should be able to draw a