312 Mr MOSELEY, ON THE 



will revolve about the points S, S', Q, Q' and A, as represented in 

 the figure 5. Now this is what is uniformly observed to take place 

 in the fall of the arch. 



12. Gauthey, having occasion to "destroy a bridge, caused one of its 

 arches to be insulated from the rest; and the adhesion of the cement 

 being sufficient to counteract the tendency of the pressure to rupture 

 the piers, he caused them to be cut across. The whole then at once 

 fell, the falling portion separating itself into four parts. Having con- 

 structed small arches of soft stone, and without cement he loaded them 

 until they fell. Their fall was always observed to be attended with the 

 same circumstances. Before the arch finally yielded the stone also was 

 observed to chip at the intrados about the points Q and Q', round 

 which the upper portions of it finally revolved. 



Some experiments made by Professor Robinson with chalk models 

 were attended with slightly different results. Having loaded them at 

 the crown until they fell, he observed first, that the points where 

 the material began to yield were not precisely those where the rupture 

 finally took place. 



This fact presents a remarkable confirmation of the theory expounded 

 in this paper. 



It is manifest, that according to that theory, with any variation 

 in the least force P', which would support the semi-arch if applied 

 at its crown, there will be a corresponding change in the position of 

 the point Q. 



Now as the load upon the crown is increased, this least force P' 

 is manifestly increased. The result is a corresponding variation in the 

 • form of the line of pressure, tending to carry its point of contact 

 with the intrados lower down upon the arch. 



This is precisely what Professor Robinson observed. The arch 

 began to chip at a point about half way between the crown and the 

 point where the rupture finally took place. 



