228 NOTES ON DOCK'S AND DOCK CONSTRUCTION. 



of opening or closing this roller runs on a metal racer or path- 

 way fixed to the entrance platform. 



To ensure contact between the roller and the pathway, 

 provision is made for adjusting by mechanical means the vertical 

 position of the roller with regard to the gate. 



As the roller-path forms part of a segment of a circle of 

 which the centre is the axis of the heel-post, it follows that the 

 outer edge of the roller has to travel a greater distance than 

 the inner edge in the same time. On this account the roller 

 should be made conical to a corresponding degree, and the axle 

 on which it turns should coincide with a straight line passing 

 through the axis of the heel-post and position of the roller. 



In the case of curved gates, 1 the usual practice is to place 

 the roller under the centre of gravity of the gate, this being 

 the position which gives the greatest steadiness in travelling 

 and the least racking and working of the joints. The true 

 position of the roller is, however, difficult to determine with 

 accuracy, especially when the gates are composed of materials of 

 varying specific gravity. 



In the case of straight gates, 2 the most suitable position for 

 the roller, and the one usually adopted in practice, is to place 

 it so that it takes half the weight of the gate, the other half 

 being borne by the gate pivot. 



Strong opinions have been expressed both for and against 

 the use of rollers. In the opinion of the late Sir J. Hawkshaw, 3 

 their only use is to relieve to some extent the pressure of the 

 heel-post against the lower part of the hollow quoin. 



Mr. Hayter considers 4 that, unless under exceptional circum- 

 stances, it would be better to abandon rollers altogether, on the 

 ground of the greater facility with which the gates can be 

 manipulated. When, however, gates are of great span, one roller 

 may be introduced with advantage. In support of this opinion, 

 Mr. Hayter 5 refers to the fact that in Holland rollers have been 

 almost universally abandoned ; and in the case of obstruction 

 occurring on the roller-path, preventing the proper closing of the 

 gates, the water would escape from the dock, and much damage 

 would result to shipping. 6 



When one of the gates at the Bristol Dock entrance, 7 which 

 had been in use many years, was removed, it was found that, 



1 M.P.I.C.E., vol. Iv. p. 117. 2 Ibid, p. 110. ' 7/,/W., vol. 1. p. 89. 



4 Ibid., vol. Iv. p. 72; vol. lix. p. 5. 5 Ibid., vol. lix. p. 1. 



6 Hid., p. 5. ' Hid., p. 11. 



