242 



On the Principles of Arches . 



same points as they would if the arch-stones had been joined 

 tvith common mortar. 



The Emerson theory supposes the arch-stones to be inflexi- 

 ble lines, which are balanced by vertical weights pressing on the 

 angles of contact ; or, what amounts to the same thing, the depth 

 of the abutting joints of the arcli-stones is not supposed to be 

 of sensible magnitude. Of this we may say, what Dr. Hutton 

 has said of the wedge theory, that " it is founded on a supposi- 

 tion which is neither in nature nor art, and which can never 

 take place in any real construction of an arch." The Emerson 

 theory certainly furnishes no practical rules for construction, 

 neither for the depth of the arch-stones nor for connecting 

 them together so that they may be less liable to change their 

 positions: but this is not the only defect it has, for it does not 

 determine the form of an arch which would be equally strong 

 throughout. 



We will take a simple case: let four equal and inflexible bars, 

 in the same vertical plane, be kept in e(iuilibrio by weights laid 

 upon the angles, the depth of the abutting joints being verv 

 small, and the points A and B fixed. Let the weight at D bl- 



