120 



THE AROH. 



must bo premised that the arch is supposed to bo built 

 with a perfect vertical bond, and not in rings, as has boon 

 too commonly and unscientifically practised. 



Now, a good and thoroughly well-burnt brindle or vitrified 

 brick is as strong as any stone, and will boar a crushing 

 weight of four or five tons on the square inch ; therefore, 

 with this material also, the cement will be the weakest 

 part of the arch, and therefore the limit of strength. But 

 tho advantage of the brick is, that it is handy, and can be 

 thoroughly bedded, quickly and without difiiculty, in tho 

 cement. This gives tho weaker material, the cement, a/w/i- 

 hedded bearing, and for that reason makes brickwork suj)erior 

 to stonework in building an arch which has to carry groat 

 pressure, even if the stones are dressed to the full thickness 

 of the arch as done by Perronet. 



Thus good brickwork forma tho strongest arch, as it is 

 also by very far the choapc-st. 



In a properly constructed arch such as has been described, 

 let us consider for a moment the limit of span. 



I have said, on a former occasion, " I think that engineers 

 have been very bold in ironwork and very timid in brick- 

 work." 



The ironwork is considered safe under a load equal to a 

 quarter of its breaking strain ; but brickwork is supposed to 

 be safe only under an eighth of its crushing weight. This 

 small limit to the safe strength of brickwork has probably 

 arisen from the false construction of the brickwork. I have 

 known several bridges to fall down from tho dislocation 

 arising from false construction, but never in any caso from 

 the crushing of the materials ; for when the arches alluded 

 to fell, the bricks were as sound as on the day when they 

 were put in. 



Comparing the two rules of safe load just mentioned, it is 



