THE ARCH. 



125 



the momont there is anj sottlemont in tlio arch or its abut- 

 ments ; the rings part company immediately, and nothing but 

 friction keeps the arch up— the real arcli principle is gone. 

 Then again, when the curve of the arch varies at all from 

 the equilibrated curve explained above, if the natural line 

 of thrust passes from one ring into the next, the arch would 

 fall at once if friction did not prevent it. In arches of 

 small span it does not so much matter, but in arches of largo 

 span it is fatal. Arches of wide span cannot be built with 

 safety in rings: for example, referring to the model, the 

 natural line of thrust must then pass round through each 

 single 4|- inch arch ; but if that arch were built in the form 

 of a circular arc, the line of thrust would, as has been stated, 

 leave the ring courses for a distance of six inches, and the 

 line of thrust would pass from one 4.| inch arch into its 

 neighbour ; and under the great thrust due to a wide span 

 the rings would slide upon one another, the haunches would 

 rise, and the crown would droop until the arch fell. Even 

 if built upon the true equilibrated curve, its stability would 

 be very precarious, so slight a margin being left for any 

 little settlement or other accidental defect. 



The almost universal adoption of the ring system of 

 buihiing arches, together with the use of unscientific curva- 

 ture in the form of the arch, with the consequent sottlemont 

 or failure of many of them, may prob.ably account for the 

 timidity of engineers in adopting brick arches of wide span. 



A serai-circular arch under a horizontal roadway is always 

 wrong ; the natural line of thrust must always pass out of 

 the arch into the backing, and if the arch stands, it is by 

 friction only and by the good quality of the backing which 

 has to sustain the heaviest part of the thrust. 



That friction is a great power there is no doubt, as is 

 proved by many instances, a few of which will be mentioned 



