1C8 



THE ABCH. 



throughout, leaving a thickness of- 12 inches as a margin of 

 safety against any moving or additional load that may bo 

 placed upon it. 



If a d liferent form of ciirve in the arch had hocn adopted, 

 that of a circular arc for example, the thickness of tlie arch 

 brickwork miist contain within it the needful 15 inches of 

 thickness all round in the true line of equilibrated thrust; 

 for Nature will follow no other, form the arch how yon will. 

 Now, the circular arc leaves the true line of thrust G inches 

 at the haunches on each side ; therefore, to make the circular 

 arch equally strong, it must be made six inclies tJdclecr all 

 round. This would, therefore, require nearly a quarter more 

 brickwork, and then the arch would not be nearly as elegant 

 as the natural curve. 



That the curve of this arch is truly equilibrated may bo 

 shown in a simple and practical manner by suspension, as has 

 been before described. 



We have hero a brass chain of the length of the cirrve 

 as shown on the model drawing. Each liidi of the chain 

 represents 9| inches of the aroh, and from each link is sus- 

 pended a steel rod of a length representing exactly the 

 calculated load upon that 9| inches of the arch. The model 

 being taken to represent one foot in breadth for the con- 

 venience of calculation.* The model is not perfect, but it is 

 near enough to the truth, to fairly represent tlio various 

 points alluded to. 



Let us now invert the drawing, and by moans of those 

 hooks hang on tho cliain, so as to bring the cliain-ends to 

 the points of springing. It will bo at once perceived that 

 the chain, as it adapts itself to tho true theoretical line of 

 equilibration, precisely represents also the curve of the arch 



* One ounce in the model reprcaonts 2J tons in the bridge. 



