224 CORNISH VIADUCTS. 



possible to avoid, and as difficult to properly accoiint for. It 

 has since been obviated by working the batter to say 1 in 57 

 instead of 1 in 60. I need hardly say that these piers, as 

 well as abutments, are magnificently built. The stones are 

 splendidty bedded and bonded together, without a sign of 

 that abominable scamping which so often ruins well-designed 

 work. 



It was, of course, impossible to use an overhead way in 

 building these piers, on account of their great height, and it 

 would have been equally absurd to use scaffolding. Steam 

 derrick cranes were therefore emplo^^ed, provided with 70 

 feet jibs, and capable of lifting a weight of 3 tons. These 

 were placed on stages 70 feet above the ground, thus 

 enabling the cranes to set stones up to 120 feet above 

 ground level. The men were also taken on and off the 

 work by means of cranes. 



When the impost courses were set, wrought iron lattice 

 girders, made on the ground out of old rails, were hoisted 

 up and placed upon them. These girders were curved at the 

 two ends to the radius of the arch, so that by means of hand 

 cranes placed upon them, the arches and backing were built 

 up to 10 feet above springing. The timber centres were then 

 placed upon the girders, the weight being transmitted from 

 the centre to the girder through sand boxes. These sand 

 boxes were introduced for the purpose of slackening the 

 centre more easily. When the arch was keyed, the sand was 

 removed through a small hole in the bottom of the box, and 

 the centre was lowered. Sand boxes were found to be pre- 

 ferable to slackening wedges, the objection to them being 

 that the loosening of the wedge blocks always causes an un- 

 comfortable jar to the work. This defect does not occur with 

 the sand box, where the centre comes away quite easily. 



Two temporary roads were constructed, one on each side 



