PASSAGE OF RIVERS.

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of the radius, thus breaking joint on the thickness as well as on the breadth of thearch.

17. Cements .—One of the best cements ordinarily to be obtained in India ismade of one part stone lime and two parts fine soorkee ; bujree or gravel being seldomfine enough for arch joints. The lime should be fresh and caustic; the soorkeeshould be made of the soundest bricks. The two ingredients should be mixed andground dry under a chuckee or grinding-stone, and should then be slaked with justsufficient water to make them into a paste. If fine bujree be used, the same mode oftreatment is to be observed. The quality of the cement depends greatly upon thelime being slaked from its caustic state whilst in contact with the gravel or soorkee.

18. Haunches .—To relieve the shoulders or haunches of the arch from unnecessaryweight, as well as to save material, thin longitudinal walls, called spandril walls, arebuilt at intervals, extending from the abutments nearly to the centre: their summitssupport either slab-stones for covering the intervals, or small vaults of masonry.

19. These walls may be or 2 feet in thickness ; their intervals, where flags areused to cover them, must depend upon the size of the stone procurable. Whenvaulting is used, the intervals may be 3 or 3'; the outer walls in this case shouldnot be under 2 feet.

SECTION VI.-ROADWAY.

On roads of great traffic, and when the bridges are small, they should be the fullbreadth of the road. But when bridges are large, this would cause them to be tooexpensive; they should not, however, be less than 24 or 25 feet in clear width ofroadway between the parapets.

2. A flat or level roadway is the most convenient, but where a rise is necessary,the slope should not exceed 1 in 35, or 1 foot of rise to 35 feet of base.

3. Above the extrados or back of arch there must be laid a course of bricks onedge, fixed in mortar, and over this a layer of 6" of metal (beat down from 9"): thissurface should be well drained by means of outlets through the parapet walls.

4. The roadway should always be guarded by parapet w r alls, varying from 3 to 5feet in height and from 1£ to 2 in thickness, according to the nature of the bridge.These walls are continued with a curved splay outwards from the end of the bridge,to form proper entrances or approaches.

SECTION VII.-TREATING OF THE FOUNDATIONS FOR BRIDGES, ESPECIALLY IN

INDIAN RIVERS.

Hitherto I have considered the abutments and piers as standing upon solidsoils, their bases being spread out to give a better footing; but in India it too fre-quently happens that this precaution is not sufficient. The general character of theearth’s crust, in India , is a superstratum or upper layer of clay, varying in quality byits mixture with sand or vegetable mould, and varying in thickness from 3 feet to 20,or even more, with a substratum of sand to great depths, but generally containingthick or thin layers of clay, or kunkury clay, lying at various depths below the sur-face of the sand.

2. When rivers run in the upper stratum of clay without cutting through it, theirstreams will generally he found to be sluggish, having little slope, and running acrossor obliquely with the general line of drainage. With such streams it will be ne-cessary merely to sink the footing of the piers and abutments a few feet below thebed of the stream; but where the clay has been cut through, exposing the sand, itbecomes necessary to take further precautions for fixing the feet of the piers andabutments.