4G8

HISTORY OF ENGINEERING.

Hook I.

more; each stone should he at least 30 inches in length, and break joint 1 foot with thoseadjoining.

“ Rubble backing of spandrills to consist of hammer-dressed rubble masonry, laid in regularcourses on their natural bed, not more than two stones in thickness for one of outsideashlar ; all properly bonded with the facing stones, as well as among the others they arelaid with.

“ Interior spandrills to be of good rubble masonry, hammer-dressed, and laid flush in goodwater-lime mortar.

“ Spandrill hearting to be built up solid between the backs of the arch-stones to the levelof the water, and composed of good hammer-dressed rubble masonry, laid in regularcourses on horizontal beds, well flushed in water-lime mortar.

“ Cross walls over each pier and abutment may be either of rubble masonry or brick.

“ Gutters and curbstones. — 'lire gutter stone of granite, 14 inches wide, and 9 inchesdeep, to have cut in it a triangular water channel, 8 inches wide and 4 inches deep ; thecurb, 15 inches deep, and 12 wide, formed also of granite, should have its outer anglerounded off, and the interior angle checked down about 4 inches, to receive and supportthe footpath pavement.

“ The gutter and curb-stones should not be less than 3 feet in length, and axed on the faceswhich are seen ; their joints being arris chisel drafted, or neatly axed, so as to make aclose joint, and the whole set in good water-lime mortar.

“ Footpath pavement should be laid in regular courses, and if possible the stones to be ofthe length of the whole width between the curb-stone and parapet wall; they should neverexceed three stones in width ; where this cannot be accomplished, the joints should alternate,and the surface be neatly broached, and made with an inclination of an inch to 3 feet, or1 in 36, towards the curb-stone, the top of which should be laid G inches above the gutterstone.

“ Road concrete. — The stone shivers and dry rubbish having been well pounded, andfilled in between the wing walls, the whole is then to be covered with a bed of lime amigravel concrete well mixed, in the proportions of four measures of clean gravel to onemeasure of water-lime mortar, used fresh. This bed of concrete should not be less than9 inches in thickness on the outside, and 12 inches in the middle of the roadway, thesurface forming a uniform curve. Upon this may be laid the paving or metal for theroad.

“ Drain pipes, of iron, 6 or 8 inches diameter, arc to be introduced to carry of!’the water,passing through the arch-stones of the bridge, and continued down the abutments.”

Number and dimensions of bridges built under the Highland Road and Bridge Act of1803, by Thomas Telford ; —

1,075 bridges of one arch, varying from 4 feet span up to 65 feet, and affording a water-way of 10,198 feet.

13 bridges of two arches, with a water-way of 643 feet.

16 bridges of three arches, with a water-way of 1236 feet.

2 bridges of five arches, with a water-way of 522 feet.

1L others with forty-three arches, and having a water-way of 2387 feet.

Making a total of 1117 bridges, 1202 arches, and 14,G8G feet of water-way.

“ General specification for the bridges _Required that they should be built over each river

or stream with stone and lime mortar, and that the foundations should be sunk to and laidon the rock, wherever practicable; where this could not be done, then the foundations wererequired to be sunk 2 feet at least below the lowest part of the bed of the river ; and wher-ever the ground w'as loose a platform of timber was to be laid under the foundations of themasonry ; this platform was to consist of two thicknesses, of 3-inch plank, laid crossingeach other, and if necessary a row of pile planking driven all round the outside. If theground was hard, instead of this timber platform, an inverted arch or pavement was to belaid and wedged between the abutments, the entire width of the bridge, and well securedabove and below by means of rows of piles sunk deeply into the bed of the river.

“ The span of the arch to be according to the table annexed. The breadth of the road-way between the parapets to be 18 feet in the narrowest part.

The parapets to be not less than 18 inches in thickness, of the height mentioned in thetable, coped with hammer-dressed stones, set on edge in lime mortar, not less than 9 inchesin depth, with a large stone at each extremity of the parapet.

“ Wherever the ground required them, retaining walls are to be used, of dry stone ofsufficient thickness, as described for breastwork, to support the made-up ground, and thesewalls to run from the extremity of the parapets into firm ground, unless the distanceexceed 20 yards.

“ When the bridges are not set upon rock, bulwarks of dry stone are to be introducedabove and below' the abutments, 10 yards in length. The dimensions of the masonry to beas set out in the table below.

“ The spandrills of the arches to be filled with stone or coarse gravel, so that the rise and