8M) Mr MOSELEY, ON THE 



Thus let ABS" (fig. 3) represent an arch having the joints of its 

 voussoirs perpendicular to the intrados as they are usually made. 



Let RQPQR' be the line of pressure, touching the intrados in the 

 points Q and Q'. It is manifest that this curve is then perpendicular 

 to the joints of the voussoirs at Q and Q, and inclined in respect to 

 those above and below these points. The inclination being downwards, 

 or towards the intrados, in reference to the former, and upwards, or 

 from the intrados, in reference to the latter. 



Hence, therefore, it appears that the tendency of the pressure is 

 to cause all the voussoirs above the points Q and Q' to slide down- 

 wards, and those beneath those points, upwards. 



And that these effects may be expected to follow the striking of 

 the centre of the arch ; the weight being then suddenly thrown upon 

 the voussoirs, and these admitting of a certain degree of motion in 

 the directions of the forces impressed upon them. 



Now this is precisely what was observed at the bridge of Nogent, 

 of the construction of which Perronet has left a detailed account. 



Three straight lines were drawn upon the face of the arch before 

 the striking of the centre, shewn in the figure 4, by the polygon 

 nmm'n', mm' being horizontal, and the other two mn and m'n' stretch- 

 ing from the extremities of mm' towards the springing of the arch. 



After the centre had been struck, the lines were observed to have 

 assumed the curved forms indicated by the dotted lines MM', MN', 

 M'N', indicating, in accordance with the theory, a downward motion 

 in all the voussoirs above Q and Q', and an upward motion in those 

 beneath those points. 



These observations have been confirmed by numerous others, and 

 especially by those (made also by Perronet) at the Pont de Neuilly. 



The sinking of the voussoirs at the crown necessarily tends to pro- 

 duce a separation of their joints at the intrados in the neighbourhood 

 of that point, and thus to cause the actual contact of the key and 

 adjacent voussoirs to take place only at their superior edges. 



