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These preliminary precautions are absolutely necessary, to enable an engineer to fix
the. best line of road, with respect to general direction, and longitudinal inclination. Without the unerring guide of actual measurement and calculation, all will be guess and uncertainty. It
may be laid down as a general rule, that the best line of road between any two points will be that which is the shortest, the most level, and the cheapest of execution : but this general rule admits of much qualification ; it must, in
many cases, be governed by the comparative cost of annual repairs, and the present and future traffic that may be expected to pass over the road. Natural obstructions also, such as hills, valleys, and rivers, will intervene, and frequently render it necessary to deviate from the direct course.
In every instance of laying out a road in a hilly country, the spirit-level is essentially necessary to show the proper line of road to be selected. The general rule to be followed in surveys is to preserve the straight line, except when it becomes necessary to leave it to gain the rate of inclination that may be considered proper to be obtained, without expensive excavations and embankments. When a devi.
ation is made for this purpose, it becomes necessary to proceed in a direct line from a new point.
Thus, for instance, if it be decided to have no greater rate of inclination than 1 in 35, on a new line of road, from A to B (Plate I. Fig. 1.), and the surveyor, when he arrives at the point a, finds a greater inclination than this, , he must incline from the direct line to b. Having then gained the summit of the hill, he does not endeavour to get back into the original straight line A B, but pursues the direct line b b, unless he is again obliged, from a similar cause, to deviate from it. This part of the survey being accomplished, it will then become necessary to examine the practicability of making a direct line of road, between A and b, instead of going to the point a.
When hills are high and numerous, it sometimes
appears, from a perambulation and inspection of the country, to be advisable to leave the straight line altogether from the beginning, in order to cross the ridges, at lower levels, by a circuitous course, in the way represented by the dotted lines A c d, in the above figure.
It constantly happens that although inclinations which do not exceed the prescribed rate can be had without quitting the straight line, the ridges may be crossed, at many feet of less perpendicular height, by winding the road over lower points of them; but the propriety of doing so will depend upon the length that a road will be increased by going round to avoid passing the ridges in the direct line. The saving of perpendicular height to be passed over by a road, though a matter of so much importance and practical utility, has not hitherto received that attention from engineers which it deserves. For this reason it has been deemed advisable to bestow much consideration on it; but, as the investigation requires minute and extensive details, and cannot be conducted with full effect, without having recourse to algebraical formulæ, it has been transferred and given in note A.
When expeditious travelling is the object. the maximum rate of inclination that never should be exceeded in passing over hills, if it be practicable to avoid exceeding it, is that which will afford every advantage in descending hills, as well as in ascending them. For, as carriages are necessarily retarded in ascending hills, however moderate their inclinations may be, if horses cannot be driven at a fast pace in going down them, a great loss of time is the result. This circumstance is particularly deserving of attention, because the present average fast rate of driving over any length of road can be accomplished in no other way than by going very fast down
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
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 a general rule, in laying out a line of new road,