550 THE POPULAR SCIENCE MONTHLY. 



naturally inclined to adopt the advice of its official experts, but Mr. 

 Eads bad faitb enougb in bis plan to propose to do tbe work at bis 

 own expense and wait for bis pay until be bad demonstrated its suc- 

 cess. It was bard to get permission to make even tbe experimental 

 application of bis views tbus so liberally proposed ; but a bill was 

 finally passed to allow bim to attempt tbe improvement of tbe Soutb 

 Pass, tbe smallest of the three, and not tbe one be bad selected, and 

 tbe depth on tbe bar of whicb was only eight feet. Tbe cost of tbe 

 work was to be five and a quarter million dollars ; only half a million 

 was to be paid after a channel twenty feet deep by two hundred feet 

 in width had been secured, anotber half million after a channel twenty- 

 two feet deep, and other sums on tbe obtaining of channels of twenty- 

 six and twenty-eight feet deptb respectively ; but, as a guarantee that 

 the maintenance of tbe channel sbould not cost more than one hundred 

 thousand dollars a year, the final million of tbe whole sum was to be 

 Mithbeld until a channel of thirty feet maximum deptb bad been kept 

 throughout during twenty years. Congress, however, deeming these 

 terms unnecessarily severe, Avith remarkable unanimity voted to pay 

 bim one and three-quarter million dollars in advance of bis contract 

 terms, after he had secured twenty-two feet depth. 



Tbe conception of tbe plan of the jetties was based upon a knowl- 

 edge of tbe fact tbat tbe Mississippi River is a transporter of solid 

 material, almost all of whicb is beld in suspension by tbe mecbanical 

 effect of tbe current, and tbat tbe quantity of tbe matter whicb it is 

 able to carry increases with the square of the velocity. Tbe current 

 of the river is caused by tbe fall of tbe water from a higher to a lower 

 level ; tbat is, by the force of gravity. Tbe element whicb resists the 

 current is tbe friction of its bed ; this friction does not follow tbe law 

 of solids, but increases or diminishes exactly as tbe widtb of tbe bed or 

 wetted perimeter of its cross-section is increased or diminished : hence, 

 if tbe stream be contracted, where it is too wide, to one half its widtb, 

 one balf of tbe frictional resistance will be gone, and tbe current will 

 be more rapid, and therefore more able to carry a larger load of sedi- 

 ment. This it immediately takes up from its own bed and thus causes 

 a deepening. Tbe result of the application of tbe jetty system to tbe 

 Soutb Pass has been a triumphant justification of its author's views. 



On tbe 8tb of July, four years after be commenced tbe work at 

 the jetties, the United States inspecting officer reported that the maxi- 

 mum deptb of thirty feet bad been secured throughout the jetty chan- 

 nel, and tbat the least widtb of the twenty-six-feet channel through 

 the jetties was two hundred feet. The balance due Mr. Eads upon bis 

 contract was then paid to bim, and tbe million tbat was to be held as 

 security for maintenance was considered as earned, and placed at in- 

 terest for his benefit. The current of the river has maintained this 

 deptb ever since. Tbe cost of tbe jetties was about balf of tbe esti- 

 mated cost of tbe proposed canal. 



