of Edinburgh, Session 1874-75., 497 
2. On a Faulty Construction common in Skewed Arches. 
By Edward Sang, Esq. 
In the course of engineering works, bridges have to be thrown 
obliquely over rivers or over roads, and thus the construction of 
oblique or skewed arches is forced upon us. The skewed stone 
arch has not grown in favour, partly from the greater skill required 
in delineating and executing the forms, partly from the fact that 
such skewed arches have given signs of weakness. Hence an im- 
pression has gained ground that there is something defective in the 
principle. 
The defect, however, of those skewed arches which I have seen 
lies entirely in an erroneous mode of construction, which, but for 
the cohesion of the lime, would result in an immediate downfall. 
The pervading principle of all good mason work is this : that the 
surfaces of each stone should be dressed square to the pressures 
transmitted by them. Now, along the ridge or centre-line of the 
skew this principle is attended to ; the beds of the arch-stones are 
placed square to the line of the roadway, that is to the line of 
pressure there. In consequence the line of the course begins to 
descend on the surface of the vault; and, in continuing tbe descent, 
the architect lays off equal distances on the curves to correspond 
with equal distances along the crown. Hence all the courses 
present equal breadths measured along the lines of pressure. 
The inevitable consequence of this arrangement is, that the bed 
of the stone becomes more and more oblique to the pressure as we 
come down on the haunch of the arch ; the mechanical effect being 
just the same as if a mason, in building a wall, were to place the 
stones off the level. The ends of tbe stones, as seen on the plane 
of the parapet, present, in this case, equal graduations, and when- 
ever we see the ends of the arch-stones equally placed, we may be 
sure that this fault pervades the whole structure. The fault is 
clearly seen on one side of the model of a skewed centering ex- 
hibited to tbe Society. 
Beginning at the crown of the arch, and descending in this way, 
the course becomes inclined to the line of pressure ; it is necessary 
to bend it gradually upwards from the course just described, and 
