1844.] 



of a Pier at Madras, 



77 



regular slopes, between which there is a much steeper one of five feet in 

 ten fathoms ; from which we must conclude that this is the point where 

 the surf begins to rise. These data agree with the observations of Mr, 

 Taylor and Mr. McKennie, according to which the exterior limit of the 

 surf in ordinary weather is fixed at 460 feet from the shore. 



The distance at which the surf breaks from the shore, as has been stated 

 in the preceeding para, is never more than 80 fathoms in ordinary- 

 weather, and according to the observations of Mr. Taylor, 133 fathoms in 

 bad weather, thus the force exerted by the surf cannot be greater than 

 that which is produced by waves 5 or 6 feet high, for when the height 

 is greater and the sea is very heavy, the surf breaks beyond its farther 

 limit, and is confounded with the breakers existing all over the roads. 

 The angle of incidence of the surf with the shore has been found by Mr. 

 Taylor to be 3 degrees, but though it should be 10 or 12 degrees, the 

 obliquity of the waves would still be so small, that their shock may be 

 considered as being perpendicular to the coast. 



Mr. Taylor has observed the height of the waves at Madras with all the 

 accuracy that this learned Astromomer brings in all his scientific observa- 

 tions, and never found it to be more than 10 or 11 feet, which gives 5 or 

 5i feet only for the height above the level of the sea. On that subject 

 Cotton says, with great correctness, at Madras, hurricanes never last 

 more than a few hours, thence there is no time for the sea to rise to any 

 great height. 5 , 



According to the observations of a very able Engineer, a stone 8 feet in 

 every dimension resting upon an even plane would resist the force exerted 

 upon it by waves 20 feet high. The weight of a stone measuring 512 

 cubic feet being 896,000 lbs, the pressure of each square foot, is 1,400 fbs 

 equal to the force exerted by the waves upon a same surface, but the 

 height of a wave at Madras being only 10 or 11 feet, the force exerted by 

 them will b.2 700 or 770 lbs for each square foot in bad weather. 



The part of the projected work, exposed to the greatest risk of injury, 

 will be near the beach, for it is upon the shore that the sea breaks most 

 violently. Against the accidents to be feared, the precautions to be taken 

 will be to sink the foundations of the masonry work, forming the head 

 of the Pier ashore, to such a depth that there will be no risk of its being 

 washed away by the sea. 



The system, after which the projected structure is to be erected, is con- 

 trived so that all its parts will have to support an equal weight and to 



