124 



THE ABCH. 



additional cost would only lie eiglitponco a yard over the 

 whole arch. The fact of the special bricks being made of 

 different breadths from the ordinary bricks is useful in 

 order to break the horizontal joints, and it besides enables 

 the men readily to distinguish them ; they call them the 

 5-inoh and 6-inch bricks, and cannot make any mistake in 

 using them. 



But in flat arches, like that shown in the model, special 

 bricks are not wanted in order to form the vertical bond, 

 for the tapering of each course can then be formed in the 

 cement. In the model bridge, for example, the tapering on 

 the whole thickness of the arch, 27 inches, amounts to no 

 more than an eighth of an inch, which may bo managed by 

 making the joint a sixteenth of an inch closer, on the center- 

 ing, and the same amount wider at the back. 



In actual practice I have never found that special bricks 

 are needed in an arch which has a curvature of 30 feet or 

 more in radius. 



Before leaving the subject of brick arches, it will be as 

 well to say a few words as to arches built in rings of brick- 

 work. This system, which has been so common in this 

 country (though not now used abroad), has been adopted 

 no doubt in order to make use of nothing but commmi bricks. 

 The origin of this system may have been, that before the 

 Excise duty on bricks was abolished there was great diffi- 

 culty put in the way of making any other than common 

 bricks ; for large or special bricks were forbidden by the 

 Excise ; but now there is no such difRculty, and there is 

 consequently no excuse for using them whore taper bricks 

 would do better. 



In a ring-built arch each ring is a soyiaralo 4| inch arch, 

 and the entire arch is made up of so many distinct 4| inch 

 arches built one over the other. This is practically proved 



