106 



THE ARCH. 



Arch," — so called because tlio curvature of tliG arch is in tho 

 truly equilibrated form, and because it will serve to prove 

 in a practical manner many points in the science of Arch- 

 building. 



The scale of this large drawing * is two-thirds of an inch to 

 a foot (or an eighteenth of the natural size). The span of 

 the arch is 85 feet, the rise 10| feet, or an eighth of the 

 span, and the crown thicknosa is 5^ foot up to the road lino. 

 The road line is horizontal. The tliiokness of the brick- 

 work forming the arch itself is throe bricks, or 27 inches ; 

 the crown thickness of 5 ft. C> in. is therefore made up of 

 2 ft. 3 in. of brickwork, and 3 ft. 3 in. of superstructure or 

 metalling, etc. 



Being an "equilibrated" arch, the natural lino of thrust 

 passes exactly along the middle in thickness of the arch 

 from one abutment to the other. 



Now, taking the weight of the material, both of the brick 

 arch and of the superstructure, to be Ij cwt., or 140 lbs., to 

 the cubic foot, the load on the crown, which is 5 ft. 6 in. 

 thick, makes the vertical load there 7 cwt. on the square 

 foot. The thrust or bed pressure on the brickwork at the 

 crown is 38^ tons on every foot in widtli of the arch, which is 

 the constant horizontal thrust, throughout the wholo arch ; 

 while at the springing, on account of the greater vertical 

 load there, it will be 44'9 tons. The thrust at any point in 

 an equilibrated arch is always the horizontal thrust multi- 

 plied by the secant o^f the angle of curvature at that point. 



Again, taking the safe load npon the brickwork to bo 

 f) cwt. on the square inch, or 36 tons on the square foot, the 

 iiecesmry thickness of the arch would be 12;} inches at the 

 crown, and 15 inches at the springing ; we may therefore 

 take 15 inches as the necessary thickness of tho arch 

 * See reduced diaKnim annexed. 



