BRIDGE. 



52.3 



*. He observes upon this Table, that these materials 

 " ' are all supposed in the dry state : That the lime 

 and puzzolano must be thrown into the measure or 

 vessel with the same degree of force. 



He denominates minion to be what falls from the 

 outside of the lumps of iron stone. 



He allows the day's work of a man for beating every 

 bmhcj of terras ; that is, two bushels of lime powder 

 and one bushel of terras. The bushel is the Win- 

 Chester level with the border. He allows iron forge 

 scales, when sifted clean from dirt and clay, as equal 

 to as much terras or puzzolano. 



His mode of working is " to mix the due propor- 

 tion of the lime and the puzzolano, the terras or the 

 minion, together in dry powder j and it will also be 

 well to have at least one third of the sand (either fine 

 or coarse), likewise dry ; put as much water to the 

 lime as, with a shovel or beater, you can bring it to a 

 paste of a moderate consistence, but rather more wet 

 than to be properly used as mortar in that state ; then 

 by degrees beat in the moist sand, and afterwards the 

 dry, bringing it to a consistence by beating after 

 every addition. The dry sand is intended to dry up 

 the superfluous moisture, so as to render the mortar 

 fit for immediate use ; and if this has not brought it 

 to a sufficient stiffness, let it lie till it is inclined to 

 fet, and then beat it up to a due consistence ; or if 

 immediately wanted, beat in a little dry lime powder, 

 always however faithfully remembering not to termi- 

 nate beating till the mass has got all the toughness 

 that you find it will acquire by beating." 



In Scotland, most of the limes harden under water: 

 that from Lord Elgin's great lime works on the river 

 Forth, from Portsoy on the Murray coast, and from 

 the island of Lismore, in the Linnhe loch on the west 

 coast : the last is the best, it slacks into a buff colour. 

 Mr Telford discovered its qualities when searching 

 for materials for the western district of the Cale- 

 donian canal, by observing in a park wall, belonging 

 to the Marquis of Tweeddale, built against spongy 

 ground, and where water oozed through, that the 

 mortar was hardest, and threw out stalactites ; also 

 that it was quite perfect on the top of the wall, where 

 worked among small stones, without copiug of any 

 sort. 



SECT. II. 



On the Foundations, Piers, Abutments, Centres, 

 Arches, Spandrels, Parapets, and Roadway. 



da- The situation, general design, and materials having 

 been determined, the next step is to prepare the 

 foundations ; and if the water is shallow and upon 

 rock, or other matter sufficiently firm, the operations 

 are very simple ; nothing bung necessary but to turn 

 the water, by means of a mound of clay, from the 

 space to be occupied by the abutments and piers suc- 

 cessively, to cltar and level the ground, and to pro- 

 ceed with the masonry : But when the w-:ter is deep, 

 and the foundation soft mud or sand, or loose gravel, 

 the difficulties frequently require all the art of the 

 most experienced engineers. Even when the founda- 

 tion is clay or rock, if the water is deep, and the 

 currents from tides or laud floods considerable, few 

 operations require more talents or attention. 



Various are the schemes which have been resorted Practi**. 

 to, in order to avoid or lessen the difficulties attending * " 

 the laying the foundations of bridges. 1. A situa- 

 tion lias been chosen in the bend of the river, to 

 which, after the bridge ructcd, a new ri- 



ver course has bren cut. '2. A side channel has been 

 formed, by which the whole, or greatest part of the 

 water, has been drawn off during the time the foun- 

 dations were put in the old river course ; and the 

 water, so drawn off, was afterwards returned to it* 

 original channel. 3. Mill weirs on the river, below 

 the proposed bituation, have had their sluices drawn, 

 or have been partly removed for a time. 4. And 

 when there happened to be no weirs, the bed of the 

 river has been deepened, for a considerable distance, 

 in order to lower the water at the intended scite. 



When the depth of the water has been reduced as 

 much as possible, it remains to be determined in what 

 manner to proceed. Anciently, (as in the case of 

 London bridge,) in deep rivers, the foundations of 

 the piers were made by merely driving piles all over 

 the space, so that their heads stood level with low 

 water ; the spaces between them were filled with 

 loose stones, and the masonry begun upon the top of 

 them ; but the piers were immense masses, and re- 

 quired to be protected by sterlings ; which, leaving 

 a very confined water-way, created a head and ve- 

 locity which tore away the bed of the river imme- 

 diately below the piers. This mode having been lonjf 

 disused, we shall proceed to consider, 



1st, Batterdeaux, or Cofferdams ; and, 



2dly, Caissons. 



Both methods have been employed with success : 

 The greatest of the modern bridges in France, that is 

 to say, Orleans and Neuilly, were constructed by 

 means of the former ; whereas, in England, the 

 bridges at Westminster and Blackfriars were accom- 

 plished by the latter mode. 



OF COFFERDAMS. 



We shall first describe the measures pursued both Qf Coder- 

 in France and Britain, in working by means of bat- dams, 

 terdeaux or cofferdams. 



The forms of these are varied according to local 

 circumstances, and the ingenuity of the persons who 

 have the direction of the work. 



1. By driving two rows of vertical piles and plank 

 piles, and filling the space between them with clay. 



2. By driving main piles, and working with strong 

 planking, laid in a horizontal position. 



3. By driving one row of guagiug piles, and filling 

 the spaces between them with pile planks driven ver- 

 tically. 



The most eminent French bridge engineers, viz. 

 Mansard, Sieur Gabriel, Gamier, Hupeau and Per- 

 ronet, in constructing their greatest bridges with 

 cofferdams, directed piles, from nine to ten inches dia- 

 meter, to be placed from three to four feet fro;: 

 tre to centre, and driven from three to six feet into 

 the bed of the river, (if composed of mud, clay, or 

 gravel,) and to rise (in their rivers) six feet above 

 low water mark. Their pile planks were nine to 

 twelve inches broad, and four inches thick ; one 

 frame contained 16 of thew pile plank;, which wer- 



