THE ARCH. 97 



arolies by the addition of ordinary and flying buttresses to 

 countervail this thrust, and in making those buttresses a 

 great ornament to their structures ; they appear in fact, to 

 have had the taste to make all the parts necessary to tlie 

 utility or stability of their buildings contributory to their 

 beauty also— a grand proof of their exquisite taste ; but 

 thoy, as has been said, can have known nothing of the true 

 theory of arch-building, and must therefore in that respect 

 have acted from precedent alone. 



Now, on the contrary, we have the moans of calculating 

 exactly all the thrusts and pressures as well as the true 

 form of the balanced arch under all required conditions ; so 

 that our experimental knowledge is confined simply to that 

 of the weight and strength of the materials which have to 

 be relied upon in the construction of any arch. 



We may now proceed, in the first place, to consider the 

 principle involved in the construction of an arch. 



An arch, at all times, is a halnnce.d structure which, when 

 correctly built, maintains itself between two, more or less 

 distant, fixed points of support, or abutments, in such man- 

 gier that all its parts shall be in perfect equilibrium what- 

 ever the weight of the arch itself or that of the super- 

 structure placed upon it may be, and however that load 

 may bo situated. 



Structurally, the theory of the arch is this : Eeferring to 

 the upper diagram (page 103), which represents the simplest 

 torm of arch — namely, a rude structure of only three stones 

 •Jotwoon the abutments. This rude arch, liowevor, in- 

 volves the whole theory and principle of the balanced or 

 ' 0(juililjrated " arch, as it is called. 



Lot us suppose the two abutments to be, say, C feet apart, 

 find that the three stones are roughly squared flagstones 

 which, placed end to end on a floor, will cover a length of 



