502 



GEEAT EASTERN. 



to distribute its pressure against the ship, it was 

 built with a series of cross timbers AA, braced 

 by shores BB. The lower sections of the en- 

 trance shafts were firmly secured to the cais- 

 son by knees and bolts. Each curved timber 

 of the caisson DD consisted of 3 segments 8 

 inches broad, 11 in. deep, strengthened by 2 

 pieces 6 in. broad, breaking joints with the main 

 timbers. The ends of the caisson, being flat, 

 were strengthened by diagonal braces and 

 knees". The planking was 4 inches thick, and 

 the whole was thoroughly calked and pitched. 

 At the suggestion of Capt. Paton, to guard 

 against leakage, in case of change of form of 

 caisson by twisting under the pressure, the 

 outer surface of the caisson was covered with 

 an old Brussels carpet secured by thin wooden 

 battens. As it was necessary that the gunwale 

 of the caisson should fit the ship, and as it was 

 not be expected that this form could be deter- 

 mined accurately, the planking was of soft 

 wood, and no diagonal bracing was used to in- 

 terfere with the-twisting of the caisson to adapt 

 itself to the form of the ship. The entrance shafts 

 F (fig. 3) were curved, as shown in the draw- 

 ing, somewhat to the form of the ship, about 6 



feet broad on the outside, lengthways of the 

 ship, by 3 feet 9 inches on the inside crosswise. 

 As it was necessary that these shafts should be 

 about 35 feet long to be 6 feet above the draught 

 line of the ship when in place, and as it was im- 

 practicable to attach them at the shipyard and 

 float them 12 miles to the ship, they were made 

 in two sections, and fastened together after the 

 caisson was towed to the ship by bolts ; a ladder 

 was formed inside of each shaft for the conve- 

 nience of the workmen. As said above, it was not 

 expected that it would be possible to fit the cais- 

 son exactly to the form of the ship ; some sort 

 of adjustable packing would therefore be neces- 

 sary" to fill up any interstices, so that the water 

 might be lowered in the caisson, and the pres- 

 sure thus applied serve to twist the caisson and 

 adapt it to the mould of the ship. In seeking 

 for such a packing, the engineers conceived that 

 the water tending to enter the joint at the gun- 

 wale might be kept out by a superior pressure of 

 water in a hose around the gunwale. According- 

 ly an endless hose (h h, fig. 2) was made to fit a 

 shallow groove on the gunwale of the caisson; 

 this groove was made by nailing battens 2 inch- 

 es thick upon the outer and inner edge of the 



FIG. 2. 



gunwale. The hose was made of 2 thicknesses 

 of heavy cotton duck coated with beeswax, and 

 was 9 inches diameter, as it was expected that 

 the crack would not exceed 5 or 6 inches. Ih 

 order to prevent the hose from being forced into 

 the caisson by the pressure of the water, tend- 

 ing to enter over the gunwale before it was 

 brought in contact with the ship, the hose was 

 secured to the outside edge of the gunwale 

 by passing bands of canvas around it and se- 

 curing both ends of each band under the gun- 



wale batten on its outer side. "While the 

 caisson was being lowered, the hose was held 

 in the groove by a cross barring of rope yarn, 

 that could be cut by the diver before the hose 

 was distended. To prevent the hose from being 

 squeezed so that the water could not pass 

 through it at any point, a rope 1 inch in diame- 

 ter was inserted in the hose while being made, 

 to afford crevices for the water to pass if the 

 hose was flattened. The hose was arranged to 

 be supplied with water by means of smaller 



