246 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



with one fitted with rollers. Instances, however, are not want- 

 ing where they have been in use for years without getting out 

 of order. 



The materials of construction may be timber, iron, or steel ; 

 practically, however, the latter are now universally adopted for 

 all large structures. 



The treatment of the masonry of the entrance will be some- 

 what different in detail, depending upon the class of caisson 



FIG. 210. 



FIG. 239. 



adopted. The faces or stops against which the caisson pi 

 are carried square across the entrance and up the sides to th< 

 required height. These faces must be worked to a true pi 

 throughout, and carefully smoothed and polished so as to ensui 

 a water-tight joint between the stone and the timber keels 

 meeting-faces of the caisson. 



A simple and efficient method of getting a true plane is to 

 stretch a tine but strong wire across the entrance, at a distance 



