36

The Cantilever Bridge, though not conforming to the present requirementsof charters, would be the better for the interests of navigation. Both bridges,however, are good enough. The spans of the Three Span Bridge would benearly 640 feet clear, which is greater than the entire width of many importantnavigable rivers.

The chief difficulty in building the Three Span Bridge is in the foundationof Pier II. This must be handled at the proper season of the year and handledby men who thoroughly understand their work: that it can be put in satis-factorily admits of no doubt. The next difficulty is the erection of the threegreat spans. There are several solutions of this. The first is to erect in theusual way on falsework, simply having more powerful machinery than is com-monly used. The difficulty of maintaining falsework here during the lowwater season of the fall months is no greater than at some other points where

£ dll!-.

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APPENDIX. E -CONTINUED.

great bridges have been erected. Another method of erection would be toerect the trusses on floating falsework at a low elevation and then lift themup by hydraulic power into position. I have looked into this matter enoughto satisfy myself that it can be done without serious difficulty. Under anyarrangement the sinking of the foundations of Piers II and III and the erectionof the three long spans must be done during the low water season.

The only real difficulty in the building of the Cantilever Bridge is themanufacture and erection of the superstructure, and the difficulties of this aresimply questions of magnitude ; the material will have to be handled in verylarge pieces, requiring powerful machinery and skillful men. As this workwill be self sustaining as it goes on, it can be done at any season of the year.

I have directed Mr. Duryea to make careful observation of the high water

and should recommend that he continue at Memphis for the present. When I■

have more full reports of the borings at the site of Pier l and other data con-cerning the action of floods, I shall have a further communication to make to

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Mr. Duryea in a letter dated Peb. 11th advises me that the west shore onthe bridge line has been cutting rapidly, this being apparently due to theaction of waves and surface water rather than!a deepening of the bottom. Ithink it might be expedient to build a short and inexpensive piece of dikework a little above the bridge line very soon, this being designed to preventfurther cutting and to form a sand bar at the site of Pier IV.

I hope to visit Memphis again within the next three weeks and shall tryto see you very soon thereafter.

Very respectfully,

Geo. S. Mobison.

S'