THE ARCH. 



121 



known that with an iron beam, if a heavy load, though well 

 within its strength, is continually put upon it and taken off 

 again time after time, tJie breaking strength of that beam 

 becomes less and less ; but in the case of a mass of brick- 

 work, if a similar load is put on and taken off ever so many 

 times, the brickwork will become no weaker, for there can 

 have been no disarrangement of its particles either by de- 

 flection or from any other cause. It would therefore appear 

 that the safe load might be taken nearer to the destructive 

 limit in m-aterials that have to bear a simple crushing 

 weight than in an iron beam which 1ms to sustain a movable 

 deflecting load, which causes the iron to bend, and which 

 must, therefore, disarrange the particles every time. But 

 taking the safe load as only an eighth of the crushing load 

 on cement, or 6 cwt. on the square inch, and taking the 

 model arch before mentioned as our example, it may be re- 

 membered that we found that an arch 15 inches thick at the 

 springing and 12;} at the crown, was suflicient for a span of 

 85 feet with a rise of one-eighth of the span. As all the loads 

 and thrusts in such an arch are in direct proportion, if each 

 dimension in the model arch were multiplied by four, we 

 should have a span of 340 feet, with a rise of 42 foot, and 

 a necessary arch thickness of 6 feet. This 5 feet thick- 

 ness of arch would also of itself leave a sufficient margin of 

 saioty for the moving load, because 5 feet is only the neces- 

 sary thickness at the qyringimj, while that at the crown 

 would be nine inches less. This at once forms a 9-inch 

 margin of safety at the crown, where the moving load, it 

 will be remembered, had the greatest effect, causing in that 

 case a deflection there of 2| inches, the deflection being 

 at the springing, and gradually increasing to 2-G7 at the 

 crown. Now the total weight of that bridge was 8,350 

 tons; but one of 340 feet s]>an would have a weight of 



