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justed to their proper depth below the bottom edge of the level, can be firmly fixed in that position by a screw.
The engineer should take care to have cross drains or culverts made under the road, to carry off all the water from the side channels and covered drains. The breadth of the roadway and foot-path will depend on the traffic and intercourse.
Road-coverings.-The best road-covering is that which lasts longest, and at the same time offers the least resistance to the force of traction. To possess the one quality without the other, would leave the road still imperfect. Men eminent for their skill and practical knowledge, recommend large blocks of stones to be set by the hand, in the form of a close firm pavement. The stones set in the middle of the road to be seven inches in depth; at 9 feet from the centre, 5 inches; at 12 feet from the centre, 4 inches; and 15 feet from the centre, 3 inches. They are to be set on their broadest edge, lengthways across the road, and the breadth of the upper edge is not to exceed 4 inches in any case. All the irregularities of the part of the road pavement are to be broken off by the hammer, and all the interstices to be filled up with stone chips, firmly wedged with a hammer, so that when the whole pavement is finished, there shall be a convexity of 4 inches in the breadth of 15 feet from the centre. The middle 18 feet is to be coated with hard stone, to the depth of 6 inches; 4 of these six inches to be first put on, and worked in by carriages and horses, care to be taken to rake in the ruts, until the surface becomes firm and consolidated; after which, the remaining 2 inches are to be put on. The whole of this stone to be broken into pieces as nearly cubical as possible, that the longest may not exceed 2 inches. The paved spaces on each side of the 18 middle feet are to be coated with broken stone or strong gravel, up to the foot-path, so as to make the whole convexity of the road
6 inches from the centre to the sides of it; and the whole is to be covered with a binding one inch and a half in thickness, of good gravel, free from clay or earth.
The foot-paths should be at least 6 feet broad, and have an inclined surface of 1 inch in a yard towards the road; its surface should not be lower than the level of the centre of the road; the side of the foot-path should be lined with blocks of stone or sods, to prevent it from being damaged by currents of water into the side channels. Roads which have not great traffic on, may be made wholly of broken stones, which will answer very well. A level bed is first formed on the natural soil: then successive layers of broken stone, laid on to the depth of 12 inches in the centre, and six inches on the sides, giving sufficient time for each coat to consolidate before another is put on. A good cover may be formed of clean gravel. The layers are laid on 3 or 4 inches thick in succession, until the thickness is about 16 inches in the centre.
In putting on fresh materials, the old road surface should be loosened with a pick-axe, to the depth of one inch at least, in order that the fresh covering may incorporate with the old road, and both form one mass.
When a new line of road is determined on, drawings should be made, showing the section and plan of the natural surface. Also drawings should be made, showing the slopes of the cuttings and embankments: the form of the bed of the road and foot-path, also the number of coats to be put on, with the thickness of each; also drawings of all the intended bridges, culverts, drains, &c. All these should be accompanied by a specification. These supply sufficient data to make out an estimate of all the expenses to be incurred.
There is nothing connected with the construction of the best line of rail-road that requires more judgment and caution, than the selection of the best line to connect two given points.
In this selection, a variety of circumstances is to be considered. One line may present numerous circumstances favourable as well as unfavourable, and in comparing this with another, connecting the same two points, which likewise presents a variety of favourable as well as unfavourable circumstances, the preference must be given to that which has the greater amount of favourable circumstances. This enquiry often requires much time, labour, and capital. It would be well to compare the expense of making each line, then the probable traffic on each, and lastly the time it would take, by the same quantity of steam power, to perform the journey by each line. To help on this enquiry, those items which are equal may be left out, and compare only those which are unequal.
For example, if the expense of making two lines were ound to be equal, then the probable traffic upon each, and the time required to perform the respective journeys, by the same amount of steam power, should be compared : The probable expense of keeping each in repair, should also be taken into account. The wear and tear, and the safety of travelling on each line, should be carefully considered. With men in business, with whom a saving of time is of great importance, quick travelling is of the greatest importance; therefore, time should be taken into
High embankments, deep cuttings, elevated viaducts,
long tunnelling, abrupt curves, and expensive bridges should be avoided as much as possible; as it is evident they add much to cost of construction, and increase the liability of accidents.
In many cases, circuitous lines are not to be preferred, such as when they pass through towns of no great importance. A direct line is to be recommended, unless where a deviation from it is sure to increase the amount of traffic, offered by the intermediate towns on the circuitous line. In many cases it is better to connect the intermediate town, though of importance, to the main line by a short branch. road. Among the laws in railway construction, are—that the whole line may be traversed by the same engines that when two lines are equal in every other respect, that is to be preferred which passes through the most populous and the richest country and that it should combine the greatest effect, produced in the shortest time, and the greatest amount of public convenience, at the least possible expense.
To find the mean value of the gradients throughout a line of railway, multiply the length of each particular line, by each particular gradient; with its proper sign, the difference between the sum of all the pos tive products. and that of all the negatives ones; divided by the length of the whole line, will give the mean value of the gradients, taking the ascents positive, and the descents negative. This is the best method of comparing the effects of the two lines for, if the length of one line, multiplied by its force of traction, be equal to the length of another by its force of traction, equal tonnage would be transported along each line in equal lines, by locomotive engines of equa! power. From this it appears the engineer may at once determine which line ought to be chosen.
M. Navier says "that the elements of comparison of different lines of railway may be divided into two heads:
1.-The establishment of a very rapid mode of transport.
2. The increase of wealth which may result from the establishment of a line of railway.
Numerous experiments have been made, to ascertain the angle of friction on rail-roads, and from the result, it appears the angle varies between 1 perpendicular to 260 in length, and 1 to 280; that is, 18.8 to 20.3 feet for one mile in length. The angle of friction denotes that inclination of a plane on which the force of gravity, parallel to the plane, is equal to the retarding force of friction: Hence, it follows, that if a body be placed on such a plane at rest, it will continue its position; or if the body be put in motion, that motion will continue uniform. Hence it follows that the inclination on a rail-road is not to be greater than 20 feet of a rise for 1 mile in length, if convenient.
The danger attending railways, when the slopes exceed a certain limit, should induce engineers to avoid them as much as possible, with due regard to economy.
Various experiments have been made, to ascertain the effects of slopes. In order to do this, it is necessary to ascertain the amount of force required to propel one ton on a horizontal line of a railway.
This is assumed by some at 8 lbs. per ton, being the mean result of repeated experiments. Now, 81⁄2 lbs. the amount of traction, being the part of 2240 lbs., the weight propelled, it follows that the traction on a level railway is theth part of the load drawn. It appears from mechanics, that the weight moved upon an inclined plane, is to the power, as the length of the plane is to the height. Therefore it follows that if a train be drawn up an inclined plane, raising 1 in 264, an addition of 81 lbs. to the traction on a level road will be required to draw one ton up such an inclined plane; that is, twice the force will be required to draw a ton weight up an inclined plane,