220 RENNIE'S DOCKS AND HARBOURS. PART VII. 



which had become more urgent than ever. Without loss 

 of time he proceeded to design and construct a bell of 

 cast iron, about 6 feet in height, 4i feet wide, and 6 feet 

 long, having one end rather thicker and heavier than 

 the other, that it might sink lower, and thus enable the 

 exhausted or breathed air more readily to escape. At 

 the top of the bell eight solid bull's-eyes of cast glass 

 were fixed, well secured and made water-tight by means 

 of leathern and copper collars covered with white lead, 

 and firmly secured by copper screw bolts. To the top 

 of the inside were attached two strong chains for the 

 purpose of fastening to them any materials that might 

 be required for the work, and flanges were cast along 

 the sides of the bell, on which two seats were placed, 

 with footboards, for the use of the men while working. 

 In the centre of the top was a circular hole, to which 

 a brass-screwed lining was firmly fixed, and into this a 

 brass nozzle was screwed, having a leathern water-tight 

 hose fastened to it, 2i inches in diameter. The hose was 

 in lengths of about 8 feet, with brass-screwed nozzles at 

 each end, so that it could be lengthened or shortened at 

 pleasure, according to the depth of water at which the 

 men in the bell were working. For the purpose of duly 

 supplying the machine with air, a double air-pump was 

 provided, which was worked by a sufficient number of 

 men. The air-pump was connected with the hose referred 

 to, and was either placed on the platform above or in a 

 boat which constantly attended the bell while under water. 

 Two stout wrought iron rings were fixed on the top of 

 the machine, to which ropes or chains were attached for 

 the purpose of lowering or raising it. The whole weighed 

 about five tons ; and it was attached to a circular frame- 

 work of timber, strengthened by iron, erected over where 

 the intended new circular pier-head was to be built, and 

 so fixed to a pivot near the centre of the work, that 

 it was enabled easily to traverse its outer limits. On 

 the top of the framework was a truck, made to move 



