Table of Experiments made by Mr. G. Rennie upon Cast and Wrought Iron. Weight applied at an arm of lever of 2 feet.

875. FROM experiments made in Sweden by a government commission it appears that both the ductility and the strength of steel and iron are influenced by the amount of carbon they

contain.

The experiments show that the hardest material has the greatest strength both before and after a permanent set has taken place from the force employed; but its ductility is also the least. The Bessemer steel in these experiments gave the same results as the other processes for obtaining steel, the same pig iron being used in each case.

The limit for the amount of carbon for the Bessemer steel is from 1.2 to 1.5 per cent. With a larger amount both the strength and ductility was found to decrease. When the amount of carbon does not exceed 0.4 per cent. the ductility of Bessemer steel is about the same as puddled iron from the same pig iron, and as it is not only much stronger but more dense and homogeneous than the puddled material, it is decidedly superior for railway purposes.

From the experiments of the same commission that the strength both of iron and steel, subjected to strains between the extremes of temperature of boiling water and freezing mercury, was greater during low than at ordinary tempera

tures.

The cheaper methods which have been introduced into the manufacture of steel within but a few years past, have brought this material within the class of the ordinary materials for engineering purposes; as railroad bars, bridges, etc.; and has led to a very extensive series of experiments upon its resistance to the usual strains on building materials; among the most noted of which are those of Mr. Fairbairn and of Mr. Kirkaldy.

The results of Mr. Fairbairn's experiments, Report of the British Association, 1867, give for the mean rupturing strain from extension 106,848 lbs. per square inch; and for compression a mean rupturing strain of 225,568 lbs. per square inch.

From the same series of experiments upon bars deflected under moderate transverse strains the coefficient or modulus of elasticity deduced was 31,000,000 lbs. per square inch. From the experiments already referred to by Mr. Kirkaldy, the following general conclusions were arrived at:

1. The breaking strain of steel, when taken alone, gives no clue to the real qualities of various kinds of that metal (74). 2. The contraction of area at fracture of specimens of steel must be ascertained as well as in those of iron (74).

3. The breaking strain, jointly with the contraction of area, affords the means of comparing the peculiarity in various lots of specimens (74, 75).

4. Some descriptions of steel are found to be very hard,

and, consequently, suitable for some purposes, whilst others are extremely soft, and equally suitable for other uses (74, 75, 78).

5. The breaking strain and contraction of area of puddled steel plates, as in iron plates, are greater in the direction in which they are rolled, whereas in cast steel they are less (74, 75).

6. Steel invariably presents, when fractured slowly, a silky fibrous appearance; when fractured suddenly the appearance is invariably granular, in which case the fracture is always at right angles to the length; when the fracture is fibrous, the angle diverges always more or less from 90° (139).

7. The granular appearance presented by steel suddenly. 8. Steel which previously broke with a silky fibrous appearance is changed into granular by being hardened (141). 9. Steel is reduced in strength by being hardened in water, while the strength is vastly increased by being hardened in oil (161, 162, 164).

10. The higher steel is heated (without, of course, running the risk of being burned) the greater is the increase of strength, by being plunged into oil (161, 162).

11. In a highly converted or hard steel the increase in strength and in hardness is greater than in a less converted or soft steel (161, 162).

12. Heated steel, by being plunged into oil instead of water, is not only considerably hardened, but toughened by the treatment (162).

13. Steel plates hardened in oil and joined together with rivets are fully equal in strength to an unjointed soft plate, or the loss of strength by riveting is more than counterbalanced by the increase in strength by hardening in oil (163).

14. Steel rivets fully larger in diameter than those used in riveting iron plates of the same thickness being found to be greatly too small for riveting steel plates, the probability is suggested that the proper proportion for iron rivets is not, as generally assumed, a diameter equal to the thickness of the two plates to be joined (163).

15. The shearing strain of steel rivets is found to be about a fourth less than the tensile strain (163).

16. The welding of steel bars, owing to their being so easily burned by slightly overheating, is a difficult and uncertain operation (181, 15).

17. The most highly converted steel does not, as some may suppose, possess the greatest density (196).

18. In cast steel the density is much greater than in puddled steel, which is even less than in some of the superior descriptions of wrought iron (196).

From experiments made by Major Wade, late of the U. S. Ordnance Corps, the following results were obtained for the crushing weights of cast iron on the square inch :—

Not hardened....
Hardened; low temper.
Hardened; mean temper.
Hardened; high temper..

.198,944 lbs.

.354,544 "

391,985 "

..372,598 "

From contracts made by direction of Mr. James B. Eads, chief engineer of the Illinois and St. Louis bridge, at St. Louis, Missouri, the staves of the arches, the pins and plates are to be of the crucible cast steel of commerce. Those parts subjected to compression are to withstand 60,000 pounds on the square inch, and those subjected to a tensile strain 40,000 pounds on the square inch without permanent set, and all must stand a tensile strain of 100,000 pounds on the square inch without fracture.

The modulus of elasticity of the steel not to be less than 26,000,000 pounds, nor more than 30,000,000.

VIII.

STRENGTH OF COPPER.

THE various uses to which copper is applied in constructions, render a knowledge of its resistance under various circumstances a matter of great interest to the engineer.

376. Resistance to Tensile Strain. The resistance of cast copper on the square inch, from the experiments of Mr. G. Rennie, is 8.51 tons, that of wrought copper reduced per hammer at 15.08 tons. Copper wire is stated to bear 27.30 tons on the square inch. From the experiments made under the direction of the Franklin Institute, already cited, the mean strength of rolled sheet copper is stated at 14.35 tons per square inch.

Resistance to Compressive Strain. Mr. Rennie's experiments on cubes of one-fourth of an inch on the edge, give for the crushing weight of a cube of cast copper 7,318 lbs., and of wrought copper 6,440 lbs.

377. Effects of Temperature on Tensile Strength.The experiments already cited of the Franklin Institute, show that the difference in strength at the lower temperatures, as between 60° and 90°, is scarcely greater than what arises from irregularities in the structure of the metal at ordinary temperatures. At 550° Fahr. copper loses onefourth of its tenacity at ordinary temperatures, at 817° precisely one-half, and at 1000° two-thirds.

and

Representing the results of experiments by a curve of which the ordinates represent the temperatures above 32°, the abscissas the diminutions of tenacity arising from increase of temperature, the relations between the two will be thus expressed the squares of the diminutions are as the cubes of the temperatures.

IX.

STRENGTH OF OTHER METALS.

378. MR. RENNIE states the tenacity of cast tin at 2.11 tons per square inch; and the resistance to compression of a small cube of of an inch on an edge at 966 lbs.

In the same experiments, the tenacity of cast lead is stated at 0.81 tons per square inch; and the resistance of a small cube of same size as in preceding paragraph at 483 lbs.

In the same experiments, the tenacity of hard gun-metal is stated at 16.23 tons; that of fine yellow brass at 8.01 tons. The resistance to compression of a cube of brass the same as before mentioned, is stated at 10,304 lbs.

X.

LINEAR CONTRACTION AND EXPANSION OF METALS AND OTHER MATERIALS FROM TEMPERATURE.

379. Coefficients of Linear Expansion.-The change of length which takes place in a bar of any material estimated in fractional parts of its length at 0° Centigrade, for a