The best situation for a bridge, as it respects a road, will evidently be that which preserves the most direct line; but, for the security of the bridge, it is desirable to have a straight reach above it, and no bend near it.

The width of a bridge between the parapets should be regulated by the traffic that is to pass over it. On turnpike roads near large towns the width should be forty feet. On those in the country thirty or thirty-six feet will be sufficient, and on parish roads, twenty or twenty-four feet.

The inclinations of a roadway over a bridge should be very moderate. They should never exceed one in thirty on turnpike roads where it is possible to avoid it, without incurring a great expense in filling for the approaches.. The number and span of the arches must depend on various circumstances, which can only be taken into consideration by the engineer on the spot; and even then much more must be left to his experience and judgment than can be derived from any precise rules as to the proper number and size of the arches.

It will be sufficient to state that the main point to be attended to in every case is that the waterway should be of ample dimensions, to allow the whole body of water to pass freely in the highest floods.

For this purpose the bridges below the site of the proposed bridge should be carefully measured, and the effects of floods upon them observed. This will be a good criterion for determining what the water-way should be of the intended bridge.

N

In making a plan and estimate of the expense of building a bridge, the point to be most fully examined and considered is how to secure a foundation sufficient to support the intended superstructure. An engineer should make accurate borings, to ascertain the nature of the subsoil; and when the slightest defect appears, piling should be adopted.

As the building of large bridges does not come within the scope of this work, directions will be here given for building those smaller bridges only which are required on all roads. The following are specifications of some of the bridges built on the Holyhead Road.

Specification for a Bridge to be built over the main Drain in Maldraeth Marsh.

"The dimensions of the bridge as well as the shape and dispositions of the various parts are described in the annexed drawings.

"The masonry of all the abutments is to be of good hammer-dressed limestone ashler from the quarry of Nant, laid in regular courses, well jointed and bonded, without pinnings in the face.

"The newels at the extremities of the wing walls are also to be built of good hammer-dressed ashler.

"The arch stones and string course are to be of good sound freestone, which may be procured below Maldraeth Bridge and be boated up.

"The parapet coping to be of stones twelve

inches in depth; the stones to be flush with the inner face of the parapet, and to project four inches on the outside. The sharp angles at the top are to be rounded off.

"All the rest of the masonry to be of good sound rubble masonry, built with stones from the Nant quarry, or others equally good.

“The whole of the masonry, except the inverted arches and water wings, is to be set in mortar composed of one part of good well-burned lime, and two parts of clean sharp sand, to be well mixed and incorporated together.

"The inverted arches are to be of sound limestone, set on edge without mortar.

"The timber for the platform to be either of oak, elm, beach, or fir.

"The approaches are to be embanked at one in thirty from the ordinary level of the road; they shall have a turf mound on each side; the breadth to be thirty-two feet in the clear. The embankments to be carried up in courses not exceeding three feet each in height, and the side slopes to be

two to one.

"The metalling over the roadway to be as described in the general specification.

"There are to be water wings built both above and below the large arch of the bridge of dry limestone, each twenty-five feet in length, built vertical at the abutments, and gradually increasing in batter till they are one to one at the extremities. They are to be founded six feet below the springing of the arch, and to be built up to the level of the

springing. They are to be three feet thick at bottom and two feet at top.

"The inverted arches are to extend twenty feet above and below the bridge."

Specification for the Bridge at Pont-y-padoc. Fifty feet span.

"A new bridge is to be built over the river agreeably to the drawings which are hereunto annexed. (Plate IV. fig. 1.) The masonry of the abutments is to be of good hammer-dressed stones laid in regular courses, well bedded and jointed, and without pinnings in the face. The wing walls, spandrells, and parapets, all (but the coping) are to be of good rubble masonry, composed of stones that are sound and will stand the weather, also without pinnings. The arch stones, the string course above the arch, and the parapet coping, are all to be of stone from the quarry above Rhydllanfair, or of equally good quality.

"The stones for the parapet coping are to be laid flat, well jointed, not less than three feet in length and nine inches in thickness. All the masonry above described to be laid in good lime and

stone mortar.

"There are to be water wings above and below the bridge, to connect the abutments with the solid rock; they are to be built as high as the springing of the arch, and the total length of the four walls is not to exceed eighty feet. These walls to be built of good lime mortar.

"The filling in between the wing walls, spandrells, and for the approaches, to be brought up in regular layers, or courses not exceeding five feet in height each.

"There must be dry breast walls on each side of the road at each end of the bridge, and parapets in lime-mortar on the top of each for the whole length of the 139 yards, excepting the space the bridge occupies. The line of the top of the parapet at the N. W. end of the bridge shall be an uniform incline from the point whence the lot commences to the wing walls of the bridge. The whole length to be metalled as per general specification."

Specification of a Bridge built over the Ellesmere Canal, on the Holyhead Road.

"The dimensions of the bridge are described in the annexed drawings.

"This bridge is to be built of good sound freestone or limestone; but the two sorts are not to be mixed, and all to be set in mortar composed of one third of well slaked lime, and two thirds of clean sharp sand, well wrought together.

"The abutments, wing walls, spandrells, water wings, and parapets, all (but the coping) are to be of good coursed rubble work; the beds and joints fairly and squarely wrought with the hammer or pick, so as not to require pinnings on the face.

"The copings of the water wings and parapets, arch, string course, and caps of newels, to be neatly