Chap. II.
COMPOSITION AND USE OF MINERALS.
693
'I'hc direct cohesion of cast-iron has been variously estimated : Captain Brown foundthat a bar 1} inch square broke with a force of 11*35 tons, and estimated its cohesion at7*26 per square inch. Mr. George Rennie found it to be 8*104 and 8*14 tons, when hemade his experiments upon a bar only | of an inch square.
A bar of malleable iron admits of considerable torsion without having its strength much
diminished, but cast-iron fractures easily when submitted to twisting.
The cohesive power of cast-iron has by some writers been estimated at 10 tons per squareinch of section ; by others at 18,000 or 19,000 pounds avoirdupois, one-third of which maybe taken as the permanent cohesive strength for practical purposes. Mr. George Renniehas given us some experiments, which he made upon the resistance of J inch iron bars, whensubjected to a wrenching force; he made use of a wrought-iron lever, 2 feet in length,having an arched head of about 60 degrees, and 4 feet in diameter, of which the leverrepresented the radius; in the centre round which it moved was a square hole thatreceived the iron bar to be twisted: the lever was balanced, and a scale hung on thearched head, the other end of the bar being fixed in a square hole in a piece of iron, andthat again in a strong vice ; it was found that \ inch bars cast horizontal twisted when9 lbs. 15 ounces were placed in the scale, and that 10 lbs. 10 ounces were required to thosebars which were cast in a vertical position, to produce the same effect.
Numerous experiments have been made upon the vertical strength of iron wires of dif-ferent diameters, but there is a great discrepancy among them; in those from ^ to ^ ofan inch in diameter, their strength per square inch has been estimated at from 30 to 40tons ; the mean strength of iron wire, which does not exceed the length of an inch indiameter, will not support more than 36 tons load.
The cohesive strength of wrought-iron being from 55,000 pounds per square inch to70,000; when it is required to find its ultimate cohesive strength, the area of its sectionmust be multiplied by the relative cohesive strength, or rather by one-third of what it issupposed to be, when applied to practical purposes.
As the transverse strength of wrought or malleable inch-round iron bars, 12 inches long,loaded in the middle, and lying loose at the ends is equal to 3152 lbs. avoirdupois, and foran inch square bar 4013 lbs., to find their ultimate strength, it is only necessary to mul-tiply this strength of the square bar by the breadth, and the square of the depth in inches,and divide the product by the length in feet; the quotient will be the weight in poundsavoirdupois.
When malleable iron is subjected to the force of tension , its absolute resistance has beenfound to vary from 50,000 pounds to 80,000 pounds per square inch of section according toits quality, and its strength to resist torsion 15,360lbs. *. this is directly as the cube of oneside, and inversely as the force applied, multiplied into the length of the lever; for if wemultiply the strength of an inch-iron bar by the cube of one side in inches, and divide theproduct by the length of the lever in inches, the quotient will give the ultimate strength ofthe bar in pounds avoirdupois, or if the bar is round, the cube of its diameter must be dividedby the length of the lever : or the diameter or side of a square bar may be found by mul-tiplying the force applied in pounds by the length of the lever in inches, dividing the pro-duct by one-third of the ultimate strength of an inch bar, when the cube root of thequotient will be the diameter or side of the square in inches, capable of resisting perma-nently that force.
In revolving shafts for machinery, the strength is directly as the cube of their diametersand revolutions, and inversely as the resistance they have to overcome. A forty-horse-power steam-engine making 25 revolutions a minute is found to require a wrought-ironshaft 8 inches in diameter; the cube 8 multiplied by 25, and divided by 40, is equal to320, a constant number or multiplier for all others; as for example, an engine 65 horsepower making 23 revolutions per minute,
9*67 inches diameter nearly.
200 is the constant multiplier or number made use of in general for all second movers, and100, for that of shafts connecting the smaller parts of machinery.
Corrugated Iron is employed in the covering of buildings, and it was first used at theEastern Counties Railway shed at the London terminus, built in 1840. The plan is arectangle, being 230 feet in length, and the width is divided into three spaces ; that in themiddle is 36 feet in width, and those on each side 20 feet 6 inches: the rows of cast-ironcolumns seventeen in each, and distant 13 feet 6 inches from one another; over theircapitals they are connected by a cast-iron elliptical girder ^ of an inch in thickness withopen panels; over this is a cast-iron gutter. The centre roof rises 9 feet, and that of theside roofs 4 feet, the springing line above the rails being 22 feet 6 inches.
The corrugated wrought-iron is of the sheets called No. 16. wire gauge, or in thicknessthe fourteenth of an inch ; and the arch is formed by curving the sheets of iron in thetransverse direction to the arches themselves, and riveting them together in the directionof their length.
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