42 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



it was to be placed. This shape was measured by constructing 

 a temporary stage of the same form as the intended shield ; the 

 vertical depth was then taken with a rod at distances of 6 

 inches apart, and thus a series of ordinates were obtained, to 

 correspond with which the bottom of the shield was cut. The 

 shield was built upon the same temporary stage in a hori- 

 zontal position ; the first, or inside layer of planks, being placed 

 close together upon templates formed to the requisite curve, 

 just in the same manner as the lagging of an ordinary centering. 

 In the next layer the planks were bent round the circumference 

 at right angles to the former, being secured at each intersection by 

 a 5-inch spike ; then succeeded another layer of planks parallel to 

 the first, and similarly secured, then another parallel to the second, 

 and so on, until a total thickness of 18 inches was attained, 

 consisting of six planks of the ordinary dimensions of 3 inches 

 thick by 9 inches wide, and 14 feet in length. This number 

 of thicknesses of plank was increased according to the depth 

 of water, the upper 5 feet consisting of only two thicknesses, 

 and an additional plank being added at each additional 5 feet 

 of depth. When completed, it was lowered without difficulty 

 into place, being loaded with sufficient ballast to counteract 

 buoyancy. When in place, it simply bore against the rock at 

 either side, there being no struts or bracing of any sort ; a few 

 pine wedges were inserted on the inner side wherever the shield 

 was not in actual contact with the rock. 



Iron Dams. In constructing the Victoria Embankment on 

 the Thames, 1 wrought-iron caissons were substituted for ordinary 

 timber dams. This method of excluding the water was preferred 

 on account of the difficult nature of the foundation, that the 

 vibration due to pile driving would have been dangerous to 

 adjacent buildings, and with the view of the lower sections 

 forming a portion of the permanent work. 2 



The caissons (Figs. 27, 28, 29) consisted of sections, in two 

 parts, half-oval on plan, formed of wrought-iron plates inch 

 and | inch thick with upright flanges at each end, so that when 

 the two halves were bolted together they formed an entire 

 section 12 feet 6 inches long, 7 feet wide in the centre, and 4 

 feet 6 inches deep. Angle-irons were bolted round the top and 

 bottom of the rings, enabling them to be firmly secured to each 

 other in a vertical position. In every case the lowest ring of 



1 M.l'.l.CR, vol. liv. p. 12. * Ibid., vol. liv. p. 31. 



