« PreviousContinue »
But when the hills are very steep, and a coachman cannot keep his time except by driving very fast down them, he exposes the lives of his passengers to the greatest danger.
How much time is lost in descending steep hills will appear from the following statement: -Suppose a hill to be so steep as not to admit of a stage coach going faster down it than at the rate of six miles an hour, five minutes will be required for every half mile: but, if the hill were of an inclination of 1 in 35, it might be driven down with perfect safety at the rate of twelve miles an hour; at which rate the time for going half a mile would be two minutes and a half, so that there is a loss of half a mile in distance for every half mile down the steep hill.
Besides the loss arising from the additional horse-power required to draw over very steep hills, there are other circumstances which make it desirable to avoid them.
In descending them, the drag becomes indispensably necessary. In coach travelling, the stopping to put it on and take it off will be the loss of at least one furlong to a coach travelling at the rate of ten miles an hour; for in slacking the pace of the horses, and before they stop, nearly one minute will be occupied.
When coachmen, to save trouble, omit to put on the drag, or, as it sometimes happens, when it breaks, travellers are liable to the most dangerous description of accidents by the overturning of a coach when going at a great velocity. Even with the drag, heavy loaded carts are always taken by their drivers into the side channels of the road to try to check their speed; and thus the channels are cut into deep ruts, or rather troughs, and the under-drains broken in, unless strong posts of wood or stone are set up, which are unsightly, and dangerous to other carriages when descending at a quick rate.
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 in as fast a trot as they can go; 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 difficulty to fast driving, either in ascending or descending. For this reason it may be taken as
never, if possible, to have a greater inclination than that of 1 in 35. Particular circumstances may, no doubt, occur to require a deviation from this rule; but nothing except a clear case that the circuit to be made 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 the Seventh Report of the Parliamentary Commissioners of the Holyhead Road, made by a newly invented machine for measuring the force of traction or power required to draw carriages over different roads, fully establish this fact.*
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 easier 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, that should always be scrupulously attended to before an engineer
See Appendix No. I. for a description of this machine.
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 rapidly away, and the expense of maintaining it in order by scraping it and laying on materials will be very much increased.
The great fault of all roads in hilly countries is, that, after they ascend for a considerable height, they constantly descend again before they gain the summit of the country which they have to traverse. In this way the number of feet actually ascended is increased many times more than is necessary if each height, when once gained, were not lost again.
As one instance, amongst others, of the serious injury which the public sustains by this system of road-making, the road between London and Barnet may be mentioned, on which the total number of perpendicular feet that a horse
Barnet is only 500 feet higher than London: and, in going from Barnet to London, a horse must ascend nearly 800 feet, although London is 500 feet lower than Barnet.*
Another instance of this defect in road engineering is observable in the line of old road across the island of Anglesea, on which a horse was obliged to ascend and descend 1283 perpendicular feet more than was found necessary by Mr. Telford, when he laid out the present new line, as shown by the annexed table :—
Another instance may be observed in the road from South Mimms to Barnet. The old
In this case the Highgate and the Hampstead ridge makes it impossible to save the whole of these 800 feet without a great circuit; but several hundred feet might be saved by a proper improvement of the present road.