148 OUTLINES OF NATURAL PHILOSOPHY. 



Some other definitions are necessary for understand- 

 ing the construction of these arches. The truncated 

 wedges of which the arch is composed, and which 

 are usually of stone, are called the Voussoirs ; each 

 course of these wedges forming one voussoir. The 

 central voussoir is called the Key-stone. The sur- 

 faces which separate the voussoirs from one another, 

 are called the Joints. The interior curve of the arch 

 is called the Intrados ; the exterior, or that which 

 limits all the voussoirs, when they are in equilibrium, 

 is called the Extrados. The Abutments, are the 

 masses of masonry, at each end, that support the 

 arch. The beginning of the arch is called the Spring 

 of the arch ; the middle, the Crown ; the parts be- 

 tween the spring and the crown, the Haunches. 

 The part of the abutment from which the arch 

 springs, is termed the Impost. 



The theorem above is sufficient for the construction 

 of arches, inasmuch as, when the curve of the intra- 

 dos is given, it determines the weight of every indi- 

 vidual voussoir. The application of it, in particu- 

 lar cases, admits, however, of being greatly simpli- 

 fied. This happens remarkably when the arch is a 

 circle, and when the joints, (which are usually made 

 perpendicular to the curve of the intrados), intersect 

 in the same point. 



230. In a circular arch, the weights of the vous- 

 soirs should be as the differences of the tangents 

 of the arches, reckoned from the crown. 



This 



