'rig in this way, upon sure grounds, the skilful engineer will trace out, and carry into effect, the greatest works upon the most economical scale. In passing over hills, it is frequently necessary to quit the straight line, to avoid too steep an inclination ; for it should be observed, that whenever the inclina tion exceeds 1 in 35, a loss of speed or increase of danger must inevitably result. If, then, it be advisable to leave the true line of direction, to preserve the best line of draught, it will become matter of consideration and calculation how much the length will be thus increased. " The great fault of all roads," observes Sir H. Parnell, "in billy 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 in crossing a range of hills, it is essential to carry the road over the lowest parts ; so in pawing a valley or ravine, the highest point should be chosen: the cuttinp in the one case, and the embankments in the other will thus be reduced to the minimum. The late Mr. Telford erected several ;Infra of great magnitude for avoiding earth embankments of too great extent. The road was carried, by means of high arches of masonry, over deep ravines or valleys: of this description is the bridge over the Mouse Water, at Cardand Craiga, on the Lanark road; the bridge over Birkwood Burn, on the Glasgow road; and the celebrated bridge over the Menai Straits, in North Wales.
In passing over marsh, or other low land, care should be taken to raise the surface of the road well above the adjacent country : it must ever be borne in mind, that not only is it necessary that the moisture of the sub-soil be carried off by drainage, but that thewurface of the road be completely exposed to the action of the sun and the wind. The meet superficial observer will have noticed, that those parts of a road shaded and overhung by trees,_are always in a worse condition than those on which the sun and wind meet no obstruction ; and that roads open to the agency of sweeping currents of air, dry after wet weather in an inconceivably short space of time.
Besides the causes for deviating from the straight line which have already been mentioned, are the more subordinate ones of towns, ornamental property, &c. These are cases which are certainly sometimes of much importance, and will deserve the consideration of the engineer ; but too much weight should not be attached to them, and the ultimate object for which the road is constructed should never be lost sight of. Let it be supposed that a road is to be formed from the city of A to the town of B: if Cis situated on the right of the line of direction, it will be matter of commercial calculation, whether the increase of traffic gained by carrying the road through C will counterbalance the loss of time and speed consequent on such deviation. With respect to passing through parks,
demesnes, &c., the circumstances wifl vary so infinitely, that it is impossible to give any rules on the subject: it may, however, be fairly observed, that what ever apparent advantages may accrue from towns, if the acclivities of a road are increased, and the line of draught injured by deviations to pass through them, the.result must be injurious in the end ; and that, necessary and just as is the protection of private property, it should never be suffered to interfere with the public good ; "for let it be remembered, that society is formed for the mutual and general benefit of the whole, and it would be a very unjust measure to in commode the whole merely for the convenience, or perhaps the caprice, of an individual," (Bateman.) As the methods of levelling and surveying for a line of roads are not generally known, we propose to conclude this part of our subject (though more properly belonging to the article SURVEYING), with a brief account of lb.: mstruments used, and the methods adopted in such cues. The levels may be taken either with a level, or a theodolite. Gravatt's improved levels, made by Troughton and Sims, are most beautiful instruments, and may be relied on for accuracy in their adjustments. The line to be taken being made known to the surveyor, the staff is held by an assistant at the point from which it is proposed to commence; the surveyor then places the instrument at a convenient distance in the proper direction, and having adjusted it to the line of collimation, or level, observes and notes down the reading on the staff called the Back Station ; he then signs to the staff-holder (if two are not employed), to take up a fresh position beyond the instrument, where a similar operation is performed, called the Forward Station ; the instrument is then moved on to a situation on the line required beyond the staff, the same adjustments are made, and reading on the staff noticed as before ; and in this manner the inequalities of the ground are correctly obtained. From the columns in the field books termed Back and Fore Station, are derived the " Reduced Levels," which laid down on paper, with the distances from station to station, give the vertical section before alluded to. The method of making a survey is so clearly explained in Mr. F. Sims's useful little Treatise on Mathematical Assinanents that we shall give the passage entire.
" When a survey is to be made for the purposes of a line of railway or turn pike road, it is necessary to delineate not only the fields through which it is contemplated time line would pan, but also one or more fields on each side, to the extent of full one hundred yards, for the purpose of admitting hereafter, if necessary, an alteration to that extent at any point on the line. The instrument usually employ on such surveys is, the prismatic compass, or else a eireum ferentor, with .a land chain.