152. Compression.—Great discrepancies appear among writers on the strength of materials, as to the compressive strength of wrought iron. Though all estimate the resistance to compression, as great as to extension, yet no one in summing up the general result of experiment, places the former at more than from 50 to 75 per cent. of the latter. William Fairbairn gives, as the relative resistances to extension and compression in bars applied as girders, 2 to 1.

We have by Weisbach56,000
We have by Rondelet70,000
We have by Hodgkinson65,000
The mean63,667
Reducing by 415,917
In round numbers16,000lbs. per square inch.

As far as practice is any guide, from 8,000 to 12,000 pounds per inch is the most to be used. The ratio of 90 to 66, seems to express very nearly the action as in the most reliable structures; which will, therefore, be adopted, or 11,000 pounds per square inch nearly. The resistance to compression is very much greater after wrought iron has been somewhat compressed.

CAST-IRON.

153. Extension.—This material is seldom used to resist a tensile force. That the tables may be complete, however, the following is given:—

By Weisbach20,000pounds.
By Barlow18,233pounds.
By Overman20,000pounds.
By Rennie18,000pounds.
By Hodgkinson16,577pounds.
By the British Iron Commission15,711pounds.
The mean18,087pounds.
Reducing by 44,522pounds.
In round numbers4,500pounds.

154. Compression.

By Weisbach109,800pounds.
By Hodgkinson107,520pounds.
By Iron Commission100,000pounds.
Stirling’s toughened130,000pounds.
Mean of Common105,773pounds.
Mean of Stirling’s130,000pounds.
Reducing by 4 for safety (Common)26,443pounds.
Reducing by 4 for safety (Stirling’s)32,500pounds.
In round numbers (Common)25,000pounds.
In round numbers (Stirling’s)30,000pounds.

155. Following are given the condensed results of the preceding figures, which may be relied upon as giving perfectly safe dimensions in practice.

Wrought Iron.Cast-Iron.
15,0004,500Tensile strength,
11,00025,000Compressive strength.