CORNISH VIADUCTS. 227 



are then attached to these, and upon the struts rest the tim- 

 ber centres, which, unlike Moorswater, are all in one piece. 

 Sand-boxes are again used to transmit the weight of the 



centre. The advantage of this double bracket is that the 

 weights of two centres, or rather of their superincumbent 

 unfinished arches, are transmitted evenly on to the pier with- 

 out tending to pull it over. Also that very little material is 

 wasted, since all the timber struts come in for use again, the 

 wrought iron only been left behind, sawn off close to the 

 pier. Generally four arches are centred at a time. Before 

 striking one set of centres, two arches are keyed, the third 

 centre is rather more than half, and the fourth centre about 

 a quarter, covered. In these unfinished arches, the side 

 nearest the arch whose centres are to be slackened, is built 

 about 5| feet farther up the centre than the opposite side. 

 By thus averaging the heights of the various portions of the 

 arches, the horizontal pressure arising from the first arch 

 when slackened is gradually reduced to a minimum, and does 

 not damage the piers. The overhead way, with its travelling 

 crane, is brought into use as soon as possible, and continued 

 over each arch as the centering proceeds. The remainder 

 of the work is then similar to that already described at 

 Moorswater. 



St. Pinnock and Largin Viaducts differ entirely from the 

 others. They are constructed, as already stated, of lattice 

 iron girders on masonry raised upon the old piers. The 

 superstructure was completely transformed in a most suc- 

 cessful manner without endangering the safety of a single 



