tribute the weight in a uniform manner over the actual width of wheel, and at the same time minimize vibration, which in the ordinary traction-engine causes considerable damage to buildings situated near the road traversed.

Attempts have been made from time to time to attain this object, and while in many cases the vibration has been considerably reduced, the rigid tire has always been conspicuous by its presence.

Boulton's flexible wheel, patented some years ago, has supplied in a great measure the requirements so necessary for the effective distribution of the weight to the surface of the road, and the elimination of vibration.

The description of this wheel, or rather the tire, may be briefly stated as follows:-The driving-wheel is 7 feet in diameter and 16 inches wide. The periphery of the wheel consists of a deep rim divided into sixty-eight cells or compartments, two in the width of tire and breaking joint with each other. Into these cells, which are about 6 inches square and 9 inches deep, hardwood blocks are fitted, with an allowance for a suitable amount of radial play. Each block, with the grain of the wood pointing outwards, is bound round, near the lower end, with an iron band. The blocks rest on an indiarubber cushion 5 inches square and 1 inch thick, or some other elastic material sufficiently yielding to absorb the jar while working. The blocks are attached to the inner rim of the tire by a bolt let into the wood block several inches; it then passes through the elastic pad and inner rim, and is secured by a nut. The recess at outer end of block is fitted with a wood plug. As the wheel revolves, the weight comes on three, four, or five blocks, the indiarubber pads are compressed while the bolts work freely in the slot of the inner rim, and the blocks adapt themselves to the surface of the road.

With this class of tire it is evident that, as four or more of these hardwood blocks bear on the road surface at the same time, the weight is distributed both transversely and longitudinally. By this arrangement the pressure on the road is considerably reduced, the weight being spread over a considerable area. In the case of a traction-engine weighing 12 tons, the pressure with three blocks in contact with the road surface only amounts to a little over cwt. per square inch.

It will be readily understood that, apart from doing any damage to the road surface, the engine is much more efficient, especially when ascending steep gradients, by reason of the increased contact surface afforded by the wood blocks. As showing that traction-engines fitted with Boulton's patent springs cause little or no damage to the roads, it may be mentioned that the Ashton-under-Lyne Town Council do not charge a travelling fee for them when they pass through that borough. When moving the confirmation of the minutes recording this arrangement, the Mayor said: "The Council would see from one of the minutes that the committee had agreed to accept, without cost, traction-engines fitted with Boulton's patent spring wheels.

They found that these wheels did little or no damage whatever, and they thought it would pay them better to admit these engines than admit other traction-engines with the ordinary wheels at the usual payment."

19. Excessive Weights on Wheels.-Heavy weights carried on vehicles with narrower tires than 4 inches cause considerable wear even on a hard dry road, and, as has been pointed out, the damaging effect during alternating frost and thaw is a very serious matter. The proper width of tire, or proper load upon any vehicle for a given width of tire, is a question which deserves more attention than is usually accorded to it.

The by-laws controlling the weights to be carried on wheels should certainly restrict these weights to about 5 cwt. per inch of actual bearing surface on vehicles without springs. This pressure is almost similar to that exerted by a 15-ton road roller which makes the road for the traffic. If the weight carried on the tires of any description of vehicle were not to exceed this standard, infinitely less damage would result to the road surface, and a corresponding decrease in the cost of maintenance would be effected. Vehicles in a state of disrepair, such as those having a slightly bent axle or an excessive play between the nave and axle, aggravate the evil, and also create an additional burden on the horses drawing the vehicles.

Sir J. Macneil stated before the Committee on Steam Carriages in 1831, that, from his experience on the road between London and Shrewsbury, he considered a wheel should have a tire an inch in width for every 5 cwt. of load. Telford was of opinion that the weight should not exceed 1 ton upon each wheel; while Macadam, speaking from his experience of the Metropolitan roads, considered that the maximum load should not exceed 9 cwt. per inch of width of tire. This latter weight is undoubtedly too much for roads even in first-class repair.

20. Effect of Springs on Vehicles.-Experiments have shown that vehicles on springs diminish the wear of roads, especially at speeds beyond a walking pace. Going at a trot, they were found not to cause more wear than vehicles without springs at a walk, all other conditions being similar. Vehicles with springs improperly fixed cause considerable concussion, which in its turn destroys the road coating. If the surface of the road be irregular, there is increased draught, and this also means additional wear by the horses' feet.

The size of a wheel has also an important bearing on the wearing of roads, a wheel of small diameter causing relatively much more wear than one of large diameter. Wheels of the common size, or about 6 feet in diameter, are considered best; they also reduce the draught for the horses.

CHAPTER II.

LAYING OUT NEW ROADS, AND THE IMPROVEMENT OF EXISTING LINES OF

COMMUNICATION.

THE selection or location of country roads is carried out by making an examination or reconnaissance of the tract of country to be traversed, so as to obtain the requisite data for the purpose of determining the best route and gradients for the proposed line of communication.

21. The most direct or shortest practicable route between two points at once suggests itself; but this, in every case, must be governed by the natural features of the surface of the country. The object aimed at is to ascertain the most favourable direction in which to lay out a road, so as to convey the traffic with the least expenditure of motive power consistent with reasonable economy in construction and in the subsequent maintenance of the road and works.

22. "Economy of motive power is promoted by low summit-levels, flat gradients (as the rates of declivity of lines of land-carriage are called), easy curves, and a direct line; but limitations to the height of summits, the steepness of gradients, and the sharpness of curves, limit also the power of adapting the line to the inequalities of the ground, and so economizing works."

"*

In a thickly populated district the new line of communication is to a certain extent predetermined by the existence of towns and villages, while in a sparsely populated tract of country greater liberty in selecting the route to be followed is permissible, as the natural character of the country alone would form the basis for selection.

In either case the engineer, in laying out the line of a new road, must be governed by the principles laid down in the preceding chapter, in so far as the gradients are concerned.

23. Reconnaissance. The general series of operations preliminary to the formation of a new line of communication are the examination or

* Rankine's Civil Engineering.

reconnaissance of the country between the points to be connected, taking note of the physical features of the country, its geological formation and sources from which materials for construction may be obtained, and the probable requirements of the district to be passed through. In this work the engineer will be greatly aided by obtaining the best and most reliable maps of the district. Flying-levels are generally taken concurrently, in order to ascertain the elevations of detached points, such as passes across ridges, and valleys, also points where structures of magnitude may be required.

24. General Principles to be observed in the Field.-In laying out a line for a new road, the following data should be carefully noted and recorded in the field-book :—

Examine the inclination of the strata, their nature and condition as to dryness.

Have the surface of road exposed as much as possible to the action of the air and sun's rays.

Cross valleys and passes at right angles.

Examine beds of rivers at proposed crossings and up and down stream, with a view to secure stable foundations for bridges, culverts, etc.

Examine sources, accesses, and distances of the supply of material for the erection of structural works, and for stones suitable for the roadcovering.

Ascertain accurately the level of all existing lines of communication, such as railways, roads, canals, and of rivers and streams.

25. Method of selecting Final Route.-It is advisable, before finally fixing the direction which a new road may take, to lay out two or more trial lines in order that a comparison can be made and it may be seen which so far complies with the following requirements. One of the principal conditions which every line of communication should satisfy is that it connects the two points by the shortest practicable route consistent with flat gradients and economy of construction. A uniform gradient can always be obtained, when there is a continual rise of the ground from the point of departure and the point where the road crosses the ridge, in a serpentine course, by following the sinuous and irregular formation of the ground. It is necessary to join the straight portions of the line at the points of change of direction by curves, concave towards the hollows, and convex going round the projecting spurs of land. A return to the straight line is advisable, and should be given effect to at every opportunity. It is also essential that a uniform ascent be obtained between the two points which are to be connected, in order to avoid unnecessary ascents and descents, thus causing a loss of motive power. Long reaches of level, especially in cuttings, should be avoided, as they add to the difficulty of drainage, and to the subsequent cost of the maintenance of the road.

26. Roads in Mountainous Districts.-In very hilly districts the route may have to be formed in a zigzag fashion in order to maintain the 'maximum' gradient. It is advisable that the curved portions (which are generally somewhat abrupt), connecting the straight sections, should have the gradient reduced so as to conduce to the safety of vehicles descending. The gradients of roads may be classed into those suitable (1) for fast traffic; (2) for mixed traffic; and (3) for ordinary agricultural transit. The first will be best satisfied with maximum gradients of 1 in 50, the second with gradients of 1 in 30, and the third with gradients of 1 in 20. These gradients should, if possible, be strictly adhered to; but under certain conditions, the physical features of a district involving great expenditure in the construction of the works, gradients for short distances may be increased to 1 in 30 for fast traffic, and 1 in 15 for agricultural haulage. Marshy or very wet soils should be avoided, owing to the difficulty of obtaining a firm foundation for the road, and such places can only be made satisfactory by an effective system of drainage, the cost of which would, in most cases, greatly exceed the additional expenditure incurred by deviating from the proposed line, even though this should entail a considerable detour.

27. Topography.-As bearing upon the general information necessary to successfully lay out a line of communication, it is advisable, when exploring a district, to observe the following positions, which are of primary importance.

The physical features of a tract of country present an undulating and irregular formation, and endless varieties of forms and combinations. On close examination, however, two classes of lines or conditions of formation are perceived to exist, which go to make up the general form of the surface of the earth. These are called ridge-lines and valley-lines.

28. Ridge-Lines.-A ridge-line is distinguished by the property that along the whole of its course it is higher than the ground immediately adjacent to it on either side; it is seldom straight or level throughout any considerable part of its length, and is generally irregular both vertically and horizontally.

The lower points or depressions on a ridge-line are termed passes; the best point to cross a ridge is generally by one of the lowest passes.

29. Valley-Lines.-A valley-line, on the other hand, is distinguished by the property that along the whole of its course it is lower than the ground immediately adjacent to it on either side; or, in other words, the ground slopes upwards from it on both sides. Being the lowest point in the adjacent surroundings, the water on the surface of the ground necessarily flows towards the valley-line and, except when it is landlocked, runs along it in streams, sometimes called 'water-course lines.' Like a ridge-line, a valley-line is seldom level or straight throughout any considerable part of its length.