CAISSONS. 



261 



ways to a minimum. The upper part is a flotation chamber, 

 from which the water is excluded when the caisson is floated 

 out of the entrance. 



To facilitate the removal of the caisson into the chamber, 

 two lines of rollers running in bearings carried by cast-iron 



boxes were fixed to the masonry in the bottom of the caisson 

 camber and recess across the entrance. 



The meeting or rubbing faces were made of teak timber, 

 1 foot 3 inches wide, secured to the inner face, and were formed 

 to follow the outline of the masonry. Abrasion of the meeting- 

 faces with the faces of the stone stops was prevented by metal 

 swells projecting about f inch from the wood, which insures a 

 clearance between all the meeting-faces when the caisson is 

 hauled into or out of the chamber. 



The folding or rising and falling platform on the top of 



