242 ^N THE STRUCTURE OF COVERED WAYS. 



height of the superincumbent wall. As far as I know, thi* 

 subject has not been matheiiiatically investi^ted in all its 

 parts, and I shatl therefore submit to the consideration of 

 your readers some propositions relating to the stability of 

 overhanging walls and of triangular covered ways. 

 Bricks over- I shiill examine those cases only, in which the materials 



hanging till employed are equal rectangular parallelopipeds, whether 

 thev meet in a ^ -^ ^ , . , t 



point. bricks or wrought stones, and m the first place I shall sup- 



pose them destitute of all friction or adiiesion, and placed 

 horizontally. With such materials, it may be shown from 

 the principles of the lever only, that a covered way may 

 easily be made, not exceeding in breadth the length of three 

 or four bricks or stones, and that the combinations, repre- 

 sented in PI. VII, fig. 1 • •?, will stand in equilibrium without 

 external support: and that if the breadth of the way be 

 equal only to the length of two bricks, it may have any 

 height of wall added over it without destroying the equili- 

 ^o support brium (Fig. 8). These combinations are however incapa- 

 thewidThmust ^^^ ^^ resisting the pressure of any considerable force, and 

 be small. the method of building in this manner cannot be generally- 



advisable; but the weight of two* bricks is supported at the 

 vertex in Fig. 9, and by extending the basis, and heighten- 

 A stronger Jng the wall at the sides, a much greater strength might be 

 "*• obtained. It is however obvious, that a wall terminated in 



this manner would by no means necessarily exert such a 

 pressure on any stones, forming a facing of the oblique sur- 

 face, as is commonly supposed in the theory of the arch; on 

 Arch turned the contrary it is plain, that an arch might be turned under 

 under it. j^^ which would be sufficiently strong for every purpose, if 



capable of supporting little more than its own weight : and 

 the same reasoning is applicable to the wall in contact with 

 Pointed arch, the lower parts of every common arch. Hence it becomes 

 often eligible to construct the arch in such a manner as to 

 be more capable of resisting a pressure near its vertex ; and 

 thus its foriti will approach in some degree to that of a 

 Arches of a pointed arch. The arches of bridges, on the contrary, have 

 bridge. ^^ support the pressure of materials, of a very different de- 



scription ; and for this reason their greatest curvature should 

 . be near the abutments. 



« , s ♦ In the next place I shall inquire into the conditions requi- 

 R.c<JuwuC8 to * 



