202 report — 1877. 



The" next topic -was the use of various explosives in the mines and the recent 

 introduction of boring machines, which he stated had proved a great success at 

 Dolcoath and Cam-Brea Mines. 



On the Eddystone Lighthouse. By J. N. Douglass. 



On the Resistance of Ships, as affected by length of parallel middle Body. 

 By William Fkoude, F.R.S. 



On a new Dynamometer for large Marine Engines. 

 By William Froude, F.R.S. 



On the Works now in course of execution for improving the Navigation of one 

 of the mouths of the Mississippi y under the. direction of Mr. James Eads, C.E. 

 By Captain Douglas Galton, C.B.^D.C.L., F.R.S. 



New Orleans is situated about 120 miles above the mouth of the Mississippi. 

 The town lies in a semicircular area formed by a bend in the river, which at that 

 part is above a mile in width and of sufficient depth to accommodate the largest 

 vessels, as well as to allow them to lie alongside of the wharves which occupy the 

 circumference of the semicircle. The river flows undiminished in width and 

 breadth for about 10S miles below New Orleans, when it separates into three prin- 

 cipal passes, which in their turn divide into many subsidiary channels, forming the 

 delta by means of which its waters reach the Gulf of Mexico. 



The point where the stream separates is called the Head of the Passes, and the 

 three principal channels are termed the Pass k Loutre, the South Pass, and the 

 South-west Pass respectively. 



The Pass a Loutre separates into five branches of various sizes. 



The South Pass gives access at about midway in its length to another channel 

 called the Grand Bayon. 



The South-west Pass runs in a continuous stream for 1 7 miles, but divides into 

 separate channels at its lower end. 



At the commencement of Mr. Eads's works, about 58 per cent, of the water of the 

 Mississippi passed through the South-west Pass, 12 per cent, through the South 

 Pass, and the remainder through the Pass a Loutre. 



A shoal-bar exists at the mouth of each of these three Passes; the depth of water 

 on each bar varies generally in proportion to the volume carried to the Gulf over 

 it. This bar is composed entirely of sedimentary matter brought down by the river. 

 The water issuing from the passes, no longer confined by banks, spreads out on 

 either side. The velocity diminishes, the sediment drops, the bar forms. The 

 central thread of the current being the strongest, and the water being the deepest 

 there, the velocity is preserved and the sediment carried out much further than in 

 the shoal water over the submerged new banks of the pass. The outer crest of the 

 bar is thus thrown out 2 J miles from the end of the land at the South Pass, and 5 

 miles at the South-west Pass. The depth on the bar at the former pass was 7£ 

 feet at mean low tide, when Mr. Eads's works were commenced. 



The depth on the bar at the South-west Pass was about 15 feet ; but it has been 

 increased recently by continued dredging, under the auspices of the United-States 

 Government, to a depth of from 16 to 17 feet. 



The depth of the bar of the Pass a Loutre was somewhere between the two. The 

 rise and fall of the tide at the mouth of the Mississippi does not exceed from one 

 and a half to two feet. 



In former years the trade of New Orleans was a large and increasing one ; but 

 the gradual introduction into commerce of a class of vessels too large to cross the 



