An inclination of 1 in 35 is found by experience to be just such an inclination as admits of horses being driven in a stage coach with perfect safety, when descending as fast as they can trot; because, in such a case, the coachman can preserve his command over them, and guide and stop them as he pleases. A practical illustration that this rate of inclination is not too great, may be seen on a part of the Holyhead Road, lately made by the Parliamentary Commissioners, on the north of the city of Coventry, where the inclinations are at this rate, and are found to present no impediment to fast driving, either in ascending or descending. For this reason it may be taken as a general rule, in laying out a line of new road, never, if possible, to have a greater inclination than that of 1 in 35. Particular circumstances may, no doubt, require a deviation from this rule; but nothing except a clear case that the circuit to be made in order to gain the prescribed rate, would be so great as to require more horse labour in drawing over it than in ascending a greater inclination, should be allowed to have any weight in favour of departing from this general rule. On any rate of inclination greater than 1 in 35, the labour of horses, in ascending hills, is very much increased. The experiments detailed in Mr. Telford's Seventh Report of the Parliamentary Commissioners of the Holyhead Road, made with a newly invented

machine for measuring the force of traction required to draw carriages over different roads, fully establish this fact.*

The following is a table of the general results of the experiments made with a stage coach, on the same description of road; but on different rates of inclination, and with different rates of velocity :

See Appendix, No. 1., for a description of this machine. Mr. Telford, in his Report to the Parliamentary Commissioners of the Holyhead and Liverpool Roads, speaking of this instrument, states, "I consider Mr. Macneill's invention, for practical purposes on a large scale, one of the most valuable that has been lately given to the public."

Mr. Babbage, the Lucasian Professor of Mathematics in the University of Cambridge, in his valuable and well-known work on the Economy of Machinery and Manufactures, in considering the injury which roads sustain from various causes, states, "As connected with this subject, and as affording most valuable information upon the points in which, previous to experiment, widely different opinions have been entertained, the following extract is inserted from Mr. Telford's Report on the state of the Holyhead and Liverpool Roads. The instrument employed for the comparison was invented by Mr. Macneill, and the road between London and Shrewsbury was selected for the place of experiment. The general results, when a waggon weighing 21 cwt. was used on different sorts of roads, are as follow:1. On a well-made pavement the draught is 2. On a broken stone surface, or old flint road

3. On a gravel road

4. On a broken stone road, upon a rough pavement

foundation

5. On a broken stone surface, upon a bottoming of

33 lbs. 65

- 147

46

concrete formed of Parker's cement and gravel 46" Mr. Macneill has improved this instrument by making the machinery of it register the force of traction, the inclination of the road, and the space travelled over.-See Appendix, No. 1., p. 344, for a description of the new instrument.

From this table it will be seen that the power required, when the velocity is ten miles an hour, to draw a carriage up an inclined plane which rises one in twenty, may be taken at nearly three times as much as is required to draw it on a level road.

The note D. in the Appendix contains a mathematical explanation of the effect of hills in drawing carriages over them.

Dr. Lardner, in the evidence which he gave before the Committee of the House of Commons that was appointed in the session of 1836" to investigate the subject of tolls and turnpike trusts in Great Britain and Ireland," made some observations on the effects of hills, which are highly deserving of attention. The following is an extract from his examination.

"A road ought to be as short as possible, consistently with some regular principle as to hills, ought it not? Yes. Now, with respect to the acclivities, there is a distinct mechanical character which attaches to acclivities, depending on their steepness. One acclivity is not more injurious than another in the mere ratio in which it is more steep than another. There are some acclivities which afford a certain compensating effect in the descent; there are others that never fully compensate for the power lost in their ascent. There is an acclivity, or an inclination, which we designate in the department of mechanical science that relates to these things by the term of the 'angle of repose ;' it is the steepest acclivity down which the carriage will not roll of its own accord-down which it will not roll by its own gravity. On more steep acclivities the carriage will roll down without any tractive power; every acclivity under that limit which will require more or less of tractive force downward. Now, acclivities which are less steep than the angle of repose give a compensation in descending for the excessive tractive force they require in ascending; that is the case with acclivities between the perfect level and the angle of repose; and I take it that that inclination should be the major limit which ought to be imposed to hills, as they are called, upon the first class of turnpike roads; the more they are under that inclination of course the better, but certainly they should never exceed it. "Can you state that acclivity in figures ?-That

will depend upon several circumstances; it will depend in some measure on the carriage, because a carriage of one structure will roll down a hill, when a carriage of another structure would not. Then it will depend upon the surface of the road; but if we take the very best class of broken-stone road surface, constructed in the best manner, so as to be as hard as can be, and a good class of carriage rolling upon it, I suppose, at a rough estimate, one in forty would be the angle of repose. I should advise the great roads not to be more steep than one in forty."

Hilly ground is not always to be avoided, as being unfit for a road; for, if the hills are steep and short, it will often be easy to obtain good inclinations, or even a level road, by cutting down the summits, and laying the materials taken from them in the hollow parts. But this must be regulated by the expense to be incurred, which is a main consideration, always to be scrupulously attended to before an engineer decides upon the relative merit of several apparently favourable lines. A perfectly flat road is to be avoided, if it is not to be raised by embanking at least three or four feet above the general level of the land on each side of it, so as to expose the surface of it fully to the sun and wind; for if there is not a longitudinal inclination of at least 1 in 100 on a road, water will not run off; in consequence of which, the surface, by being for a longer time wet and damp than it otherwise would be, will wear