188 Mr. White's Design for the New Lotidofi Bridge, 



power ; and the Pozzolano the greatest disposition to indurate 

 with time. 



The third mode of construction, viz. that of Bond, must de- 

 pend upon llie strength, durability and form of the stone, as 

 well as the workmanship. If the work is laid in horizontal 

 beds, paralielopipedons of the proportions of 5, 10 and 2 will 

 make excellent bond ; and the same forms laid vertically will 

 answer for the arches, if radiated. It must be evident, how- 

 ever, that these stones, whatever their sizes may be, must 

 have evenly tooled surfaces, or cement, to make the joints in- 

 compressible after the stones are set; perhaps where the stones 

 are vertically p]aced,j^/H/ dowels ought to be employed. 



In order to apply the preceding observations to the designs, 

 it may be proper to explain that the elevation A is drawn 

 with little distinction in the stone-work, upon the presump- 

 tion of our possessing a proper cement, and that there is no 

 difficulty in obtaining a sufficient quantity, which indeed ap- 

 pears unquestionable, if Parker's or Lord Mulgrave's be em- 

 ployed where immediate adhesion is required, and Pozzolano 

 where time for setting can be allowed. None of those cements 

 will shrink ; they attach well to each other, to stone, and to 

 brick; so that if the stone or brick-work is well set and fitted 

 in, an almost homogeneous structure will be obtained. Ad- 

 mitting this to be the case, the form of the edifice itself can 

 only be so far important as that it should aftbrd sufficient 

 substance to be united capable of carrying its own weight, 

 and the adventitious weight which public traffic may impose 

 upon it ; the shape and size of the united materials not being 

 of much importance, provided they do not approach to re- 

 gular squares, or that rounded bodies are not introduced in too 

 large proportions. Perfectly vitrified brick may be equally 

 employed with stone, but not to form the external face, which 

 is supposed to have a moderately tooled plain-work surface, 

 and the arch-stones following the curve as secants to the cir- 

 cumference. 



The second design is upon the third mentioned formation, 

 viz. that of Bond, and is arranged so as to permit an inter- 

 weaving of the stones without endangering their being crushed 

 by the incumbent weight. It is obvious that the thinner the 

 stones are in their beds, the greater will be the bond ; and 

 the better tooled and the fairer the surfaces where they meet, 

 the less likely will the stones be to ride, and consequently to 

 be broken*. 



Much 



* Other modes of obtaining bond have been cniployetl, besides that 

 derived from the form of stones ; particularly the introduction of metallic 



plugs 



