866 Teaxsactioxs of the American Institute. 



The orcliuaiy caisson has a strong bottom, which is arranged to 

 form the foundation for the masonry, and generally the sides are 

 arranged to be removed after the pier has been completed to the 

 level of the water. The sub-foundation is either the natural bed 

 of the river, dredged to an uniform level plane, or formed by long 

 bearing piles, driven from above the water, and cut off to a level 

 plane at the natural or a dredged bed of the river. 



The caisson is made either of wood or of iron, and is directed 

 in its descent upon the sub-foundation by guide piles. Sometimes 

 the caisson is loaded and sunk directly to its place, and the masonry 

 built up within it, but more frequently it is allowed to float, and 

 descends as fast as it is loaded with the masonry which is built 

 within it. In some cases the descent of the caisson is regulated by 

 large screws, by means of which it is only buoyed up by its floata- 

 tion in pai-t, and enough of the weight is retained upon the screws, 

 to avoid the effect of changes in the level of the water. 



Another method is precisely similar to that just described, except 

 that there is no caisson, and only the strong platform, which is 

 arranged to serve for the permanent foundation, upon which the 

 masonry is built, being retained above the water by screws, and 

 gradually lowered as the masonry is built up. 



When a diving-bell is used, the foundation is prepared as before 

 •otated, and the stone are laid as in work above ground. As no 

 cement can be used in this case, the stone are cut to fit each other 

 closely, and are dowelled and bolted together. 



The use of iron piles will in most cases allow the work to be 

 constructed without the aid of these submarine contrivances, because 

 they may be di'iven to almost any depth, and of a diameter suffi- 

 cient to give the requisite lateral strength, and may, therefore, be 

 brought up to the surface of the water. Wooden piles, on the 

 contrary, can be no larger than the trees that produce them. 



A popular objection to the use of cast-iron piles has ai'isen from 

 an apprehension that the metal will rapidly corrode when immersed 

 in water, or exposed to the alternations of moisture and dry atmos- 

 phere. 



I have recently written a paper on this subject, which I have 

 been requested to read in this connection, as follows: 



CORROSION OF IRON. 



The question of the durability of iron, especially when exposed 

 to the alternations of moisture and the atmosphere, has been fre- 



