The height of the tower being one hundred feet, and the angle at the tower 70° 12′, we have
| Sin 19° 48′ | 9,529,864 |
| Sin 90° 00′ | 10,000,000 |
| log height (100) | 2,000,000 |
| log distance (295.2) | 2,470,136 |
which double, and we have 590.4; finally, add twice the breadth of the tops of the towers, and the whole length of cable is, from anchorage to anchorage,
1021.38 + 590.40 + 16 = 1627.78 feet.
The formula for the maximum tension, (that at the point of suspension,) is
T = ph
2f√h2 + 4f2,
which becomes
T = 4500 × 500
180√250000 + 32400 = 2966 tons.
Number 10 iron wire (20 feet per lb.) will support 1,648 lbs. per strand; this is the ultimate strength; the maximum load for safety is 400 lbs. per strand; whence 2,966 tons, or 6,642,500 lbs. will require 16,606 strands; and if we use two cables, each must have 8,303 wires; or four cables of 4,151 each. The permanent load on suspension bridges should never be more than one sixth of the ultimate strength; one eighth is a good standard. The accidental load should never exceed one fifth of the whole strength of the cables. The permanent weight supported by the Niagara bridge is only one twelfth of the ultimate strength of the cables.