522 



BRIDGE. 



Practice, which has previously been made up in heaps in the 

 / ' common way. In the specification for the Pont 

 Royal des Tliuilleries, by Mansard, and which seems 

 to have served as a model in France, it is provided, 

 that all ihe mortar of lime and cement, for the work 

 under water, shall be composed of five parts, three of 

 which shall be of good cement of tile, not bricks, 

 and two of lime of Melun, well ground, or pounded 

 together. For the common mortar, two of lime and 

 three of sand. The Sicur Gabriel directs the same 

 preparations for the bridge of Blois. Perronet, for 

 the bridge of Ncuiily, directs that the cement mortar 

 shall be equal quantities of ground tiles from St Ger- 

 main, or St Cloud, and lime ; observing that this, on 

 account of the quality of the lime, exceeds by -f^ the 

 usual quantity employed in tbis sort of mortar. Of 

 the other, or white mortar, he directs one third of 

 slacked lime, and two thirds of sand. In the account 

 of building the Eddystone light-house, Mr Smeaton 

 gives a chapter upon water cements, from which we 

 shall extract a few particulars ; and earnestly recom- 

 mend those who wish to be fully informed on this 

 point, to peruse with attention the whole of that va- 

 luable chapter. 



Mr Smeaton found, in many parts of England, 

 limestone which produced lime, that when made into 

 mortar grew hard under water. The best kind was at 

 Aberthaw, in the Welsh side of the Bristol channel. 

 He found the stone of Watchet, a small sea port in 

 Somersetshire, had long been used in water works : 

 it did not suit the purposes of agriculture. He traced 



the same sort of stone through the counties of Mon- practiei 

 mouth, Gloucester, Worcester, and Leicester, and v*" 

 thence by the vale of Belvoir into Nottingham, and 

 Lincolnhire, at a place called Long Benington : he 

 found it also in the counties of Dorset, Hants, Sus- 

 sex, and Surrey. It has also long been procured in 

 Lancashire, under the name of Sutten lime. 



After mentioning the shape, appearance, and qua- 

 lities of the sundry stones, he concludes, that as this 

 sort of limestone is found, blue, grey, yellow, and 

 white, also in thin strata and lumpish masses, some- 

 times very hard and sometimes comparatively soft ; 

 that its fitness does not depend upon those appear- 

 ances and qualities, neither does it upon the matrix 

 in which it is formed. But he found, that when 

 burnt, all the water limes fell into a buff-coloured 

 tinge, and all contained a considerable portion of 

 clay. 



He found once a reddish coarse deep brown sand 

 stone, of a somewhat tender nature, which when 

 burnt, pounded and sifted, and mixed with slacked 

 lime, and made into a ball, became very hard. 



He states, that limestone in general loses about J 

 of its weight by burning j that slacked lime produces 

 double the measure of the burnt stone or shells ; that 

 when made into a paste, it occupies but half the space 

 of the powder; that two measures of slacked lime 

 made into a paste, and mixed with one measure of 

 terras or puzzolano, makes about 1-,^. of mortar. 

 The following Table is the result of his observations, 

 experiments, and practice. 



