Imagens das páginas
PDF
ePub

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.

HILLS.

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 deviation is made for this purpose, it becomes necessary to proceed in a direct fine 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 \ 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 by which 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 upon it; but, as the investigation requires minute and extensive details, it has been transferred and given in note A.

When expeditious travelling is the object, the rate of inclination that never should be exceeded in passing over hills, if it be practicable to avoid so doing, 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 coach driving over any length of road can be accomplished in no other way than by going very fast down the hills. 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.

Few travellers by stage coaches are aware of the risk they run of losing their lives in descending hills. A coachman must be thoroughly well skilled in his business, naturally cautious, and at all times sober. The wheel horses must be not only well trained to holding back, but very strong. If a pole breaks, or a pole-piece or a haime, haime strap, or drag chain, when a heavily-laden coach is descending a steep hill, at a rate exceeding six miles an hour, an overturn is almost inevitable, by reason of the coach overpowering the horses, Hence it is that ninety-nine out of every hundred coach accidents which happen, are on hills. Nothing is more important for all turnpike trustees to pay immediate attention to, as the reducing of all hills to inclinations of at most 1 in 24.

How much time is lost in descending steep hills will appear from the following hypothesis:—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 may 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 horsepower required to draw a carriage 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, putting on the shoe, taking it off, and getting the horses again into their regular rate of going, nearly one minute will be occupied.

Even with the drag, heavily laden carts are always taken by their drivers into the side channels of the road 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 descending at a quick rate. Various plans have of late been proposed for the better applying of drags to carriages, but the only effectual plan for preventing accidents is to lower all hills to rates of inclination not exceeding 1 in 24.*

* Extract from a Letter from Sir J. Robison, Secretary of the Royal Society of Edinburgh, respecting the French drag.

"9, Atholl Crescent, Edinburgh, 5th Nov. 1836.

"Since I had the honour of an interview with you, I have passed twice between Edinburgh and Newcastle in the Chevy Chace coach, to which the French drag has been applied during a considerable period.

"I paid close attention during these journeys to every circumstance having any relation to the working of this apparatus, and I have been led to infer that important advantages would arise from the general adoption of this drag for the public conveyances of this country.

"1st A saving of all the time now lost in applying and removing the shoe.

"2d. A gain of time, with some relief to the horses, by taking advantage of a portion of the impetus acquired in going down one hill to assist in overcoming its opposite ascent, instead of being obliged, as at present, to strain the horses in checking the speed, and in backing the coach off the shoe.

"3d. In obtaining a command over the horses, by the power of increasing the resistance by a few turns of the winch."

This drag has been applied to coaches on other lines of road in Scotland with complete success.

Messrs. Makepiece and Pearson have invented another kind of drag, called the Patent Retarder : this has been for some time applied to the Devonport mail, and is considered by competent judges to answer effectually.

[ocr errors]
« AnteriorContinuar »