THE MADRAS HARBOUR. 



63 



barriers have been made upon both systems ; the sea has 

 asserted its supremacy over other examples of both kinds. 

 By common consent theory has been pronounced to be inap- 

 plicable in its present imperfect state. 



What an engineer now looks at is whether he can make 

 one or the other kind of structure cheaper, fulfilling the 

 necessary conditions of stability. Generally speaking, where 

 stone is plentiful and cheap he will adopt a slope of it for the 

 sea to spend itself upon ; where it is expensive, and his object 

 is to use as little of it as possible, he will adopt the upright 

 wall. In the case of Madras the latter system was adopted 

 simply because its cost was found to be about half that of 

 the slope. All the stone in either case would have to be 

 brought many miles by railway at a certain cost per ton, 

 and it was cheaper to take a few tons and convert them into 

 materials for an upright wall, than to convey a great many 

 more tons and throw them in their natural state into the 

 sea. 



It will be easily understood that in order to build an up- 

 right wall without mortar — and it is impossible to use mortar 

 under water or for some distance above water,— the stone 

 must be converted into such regular shapes that there shall 

 be only small interstices between them. Formerly the plan 

 was to cut the stone itself, but now for sea- work the almost 

 universal plan is to form the rough stone into concrete blocks, 

 which can be made of any required shape and size. When 

 natural stone was used, it was rare to get a block of 10 tons 

 weight fit to be squared, and 2 or 3 tons was more like the 

 average. The weight of the Madras blocks is 27 tons, and 

 about the same weight is adopted at several other similar 

 works ; but in some cases much larger blocks have been used, 

 and at Dublin blocks of 350 tons have been made above 

 water and conveyed to their place in the work and deposited 

 by specially devised machinery. 



