3H NOTES ON DOCKS AND DOCK CONSTRUCTION. 



figures by no means form the limits, as it is obvious that no 

 special engineering difficulties attend the introduction of greater 

 or less inclination. 



The character of the foundation to be adopted will depend 

 upon the nature of the ground ; a point of great importance to 

 be observed being that it should be perfectly rigid, and 

 uniform stability throughout. 



Messrs. Lightfoot and Thompson remark l that piles should 

 never be resorted to except in cases where they are absolutely 

 necessary, and even then only with great caution, unless the 

 whole length of way is to be piled. 



When part of the way is supported on piles, and part on the 

 natural ground, a difficulty is very likely to arise at the junction ; 

 for the ground being more or less yielding, the cradle, when 

 passing into the more rigid piled portion, is subjected to 

 excessive local stresses which may probably cause breakage of 

 rollers, carriage, or rails. What is to be aimed at is, to have a 

 uniform support for the ways throughout their entire length. 

 Where the foundation of the ground varies, the desired uni- 

 formity may be obtained by excavating the defective portions, 

 and filling in with suitable material. 



In order to obtain a sufficient depth of water over the cradle 

 when run down, it is necessary to carry the ways a considerable 

 distance into the water. For this purpose, divers must be 

 employed to a great extent, except in situations where the range 

 of tide is such as to admit of the lower portion of the slipway 

 being constructed during low water. 



When it is necessary to resort to the aid of divers in con- 

 structing the lower submerged portions of a slipway, the work 

 is generally carried out in the following manner : 2 The rail- 

 bearers are first of all fastened together on land, on a continuous 

 platform of transverse sleepers. The rails are then fixc-1, 

 the land end of the bearers being properly scarfed to join the 

 last length laid from the shore. The platform is floated out as 

 nearly as possible over the position it is intended to occupy, the 

 bed having been previously prepared. Guide-piles being driven, 

 the platform is loaded with stones and sunk, after which tlio 

 ends are keyed to the shore timbers, and the weights which 

 are to be permanently left, are adjusted, sights being tak- n 

 from time to time from the top of the way to check the 

 1 M.P.LC.E., vol. Ixxii. p. 137. Ibid., p. 139. 



