When the chain was again pulled for the second and third time no further permanent set was observed, and the extensions of the chain were as follows, being the averages of a considerable number of observations. It will be noted that the average extensions are not exactly proportional to the increments of stress, They are, however, given as observed.

Fig. 421. Scale ½ inch = 1 foot.

Fig 422. Scale 3 inches = 1 foot.

Table No. 39. Observed Extensions Of Hauling Chains Under Tensile Stress. (10" Links.)

The modulus of elasticity for a well-constructed chain of this type, with carefully fitted connections, and as little play between pins and holes as good workshop practice will admit of, within a range of from zero to 7.6 tons per square inch, and composed of links with a length of 10 inches centre to centre of pins, is consequently about 8,950,000 lbs. when the preliminary stretch has been taken out of it, and when the stress on the chain is about one-fourth of the ultimate strength of the material.

Fig. 423. Scale 3 inches = 1 foot.

A chain of similar construction, 9 inches from centre to centre of link pins, used in the hauling of caissons of smaller dimensions, and of which details of the links are given in Figs. 422, 423, tested in a manner similar to that above described, gave the following results:-

Table No. 40. Observed Extensions Of Hauling Chains Under Tensile Stress. (9" Links.)

The permanent set on the first application of the load was about 0.00102 of the length of the chain (111 to 118 feet). The modulus of elasticity derived from the second and third applications, the preliminary stretch being taken out of the chain, is about 8,750,000 lbs. for a chain of 9.inch links, centres of pins, or somewhat less than before.

The total mass to be moved in this case was about 750 tons, including ballast, and the chain was designed for a working pull of 50 tons on the two chains.

The elasticity of long chains of this type should be carefully borne in mind in the design of all machinery details liable to be affected by the extension under load.

The attachment of the hauling chains to the bodies of the caissons is effected by means of a yoke girder. This yoke girder, which travels on wheels upon the rectangular steel bars shown in section in Fig. 421, is attached at its centre to the caisson by a massive steel hauling bar, designed to act either in tension or compression, for pulling or thrusting, and provided with india. rubber washers to mitigate shock, while provision is made for the fleeting of the caisson from side to side. By these means the pull on the chains is equalised, the connection of the yoke girder with the chain being shown in Figs. 424, 425, a massive steel forging attached to each end of the yoke girder engaging with the hauling chain in the manner shown by dotted lines.

The following table (No. 41) gives the weight of the bodies of sliding caissons, exclusive of machinery and ballast, and inclusive of machinery but exclusive of ballast, derived from recent examples.

Figs. 424, 425. Scale ¾ inch = 1 foot.

The machinery referred to is only that included in the bodies of caissons themselves, principally connected with pump-work, valves, etc., and has no reference to any portion of the machinery, such as engines, hauling chains, or other gear, connected with the hauling of the caisson into or out of its position across the dock entrance.

This latter machinery, being fixed on land, has no influence on the design of the caisson as a floating body. The weights given are inclusive of all riveted steelwork and other ironwork, timber, paint, asphalte, cements, or other material.

Examples Nos. 4, 5, and 12 are provided with sluices to aid in the regulation of water level on a rising or falling tide alluded to on p. 406.

Table No. 41. The Weights Of Sliding Caissons