ton side, where the abutment will be located, rises 7 degrees. The distancefrom the centre of one abutment to the centre of the tower is 788 feet. If,therefore, the above grade of 4 1-2 degrees was continued the whole of thisdistance, we should overcome an additional rise of 61 feet, and therefore attainan elevation of 61 plus 80, or 141 feet above low water. In place of this, thegrade of the bridge is reduced from the abutment to the tower gradually, untilit approaches the tower at a level, and at an elevation of 122 feet above lowwater. The two spans, therefore, will sweep across the river in a curved line,which will improve the graceful appearance of the work.
It is certain that the wants of the navigation do not require an elevation ofthe floor of 122 feet. But-it has been assumed here as a maximum, in case itshould be considered necessary to resort to it. The estimates are based uponthe above dimensions, and would, of course, suffer some reduction, if the eleva-tion of the floor could be reduced.
10. PRACTICABILITY OF PROPOSED WORK.
It is the duty of the architect and engineer, when he is charged with thedesign of public works, to report previous to their execution, fairly, accurately,and candidly. Honesty of design will, next to knowledge and experience, mostsurely guarantee his professional reputation. This honesty will best be provedby full and candid explanations, exhibiting the weak as well as the strong sidesof the proposed work. I intend to pursue this course in relation to the Cincin nati bridge.
It is conceded, in professional reports, that the practicability of a work isproved by a scientific demonstration. No works on Engineering, perhaps,afford as good an opportunity as a k report on a suspension bridge, to make adisplay of scientific attainments. It is certainly worthy of remark, that nobranch of engineering has derived as much aid from science as the art of con-structing suspension bridges. As the report before us, however, is strictly apopular one, mathematical demonstrations must remain excluded. But on theother hand, it will be impossible to establish the practicability of a work of thiskind, satisfactorily, without some demonstrations. I will, therefore, attempt todiscuss the subject matter in as plain a manner as its nature will admit.
The security of the structure, and its ability to resist all forces which may bedirected against it, will principally depend upon the
a. Adequate strength of the Cables, Stays, and Suspenders.
c. Solidity of Foundations, and the
d. Stability of the Centre Tower.
These various conditions of security will be considered in the followingparagraphs:
a. Adequate Strength of Cables, Stays, and Suspenders.
A bar of good charcoal iron, 1 inch square, as has been mentioned before,will bear a weight of 60,000 pounds; its weight is 3.38 pounds per foot run. Abar of the same size, and 60,000 pounds weight, will therefore measure 17,751feet. From this it follows, that if we had a point elevated 17,751 feet, to whicha bar of that length could be suspended freely, it would bear its own weight.