As cast and wrought iron are used in all heavy framing, a few pages of this work will be devoted to a consideration of its nature and properties.

Iron is a metal found in nearly all parts of the world. Its specific gravity is .7632; being, with the exception of tin, the lightest of all metals: and it differs from them all in the fact, that, while they are made brittle by the action of heat, its malleability is thereby greatly increased.

Iron shrinks so much in cooling, that a pattern for castings should be made an eighth of an inch larger per foot than the piece is required to be when cooled. It is heated so as to appear red in the dark at 752° Fahrenheit; and, in twilight, at 884°. It is made visibly red-hot by day at 1,077°, and is thoroughly melted at 2,754°.

Cast iron expands 1/162000 of its length, in each direction, for every degree of heat; and its greatest expansion is 1/1170 of its length in the shade, and 1/1000 of its length when exposed to the sun. It will bear an extension of 1/1200 of its length without permanent or serious alteration.

Wrought iron expands 1/143000 of its length for each degree of heat. It will bear an extension of 1/1400 of its length, and a pressure of 17,800 pounds to a square-inch, without injury. Its cohesive power is diminished 1/3000 by every degree of heat.

The resisting power of cast iron has been greatly overestimated. The best experiments show that a force of 93,000 pounds to a square-inch will crush it, and that it will not bear more than 15,300 pounds without visible alteration.

The tensile strength of wrought-iron rods has been tested in a variety of ways. It has been decided that no particular amount can be named as the actual strain a rod will resist, as it has been repeatedly proved that no rod is to be depended upon as uniformly perfect throughout, a lesser strain often parting a rod of larger diameter. The cohesive power of cast iron is set down by most authors at 40,000, and of wrought iron at 60,000, pounds to a square-inch. A vertical rod, having a weight suspended at the lower end as in the case of rods supporting a tie-beam, not only supports the weight at the end, but must, in addition, sustain its own weight from the point at which it is suspended; so that a long rod will part near the upper sooner than the lower end. A perfect rod, therefore, decreases in strength as it is longer, and vice versa. The iron-work in the examples of framing given in this work is so figured as properly to support the work, and, at the same time, prevent unnecessary vibration.

The following table shows the weight of a square-foot of cast or wrought iron plate, from a sixteenth of an inch to an inch in thickness, advancing by sixteenths: -

The following table shows the weight of a foot in length of wrought or cast iron, either round or square, from half an inch to three inches in diameter, advancing by eighths: -

A cubic-foot of cast iron weighs 450.5 pounds; and one of wrought, 486.8. A cubic-inch of each weighs respectively .260 and .281.

The accompanying table shows the weight of bar-iron from a quarter of an inch to an inch in thickness, and from one to four inches in width, advancing by an eighth: -

The weights in the foregoing tables are those of English iron. American iron is a seventieth heavier; ' and therefore, in making calculations of its weight, one pound should be added to every seventy pounds as computed by the tables.

To ascertain the weight of any piece of cast iron, we have but to determine the contents in cubic inches, and multiply it by the decimal .260; or in feet, and multiply by 450.5. If it be of a shape or form that will readily admit of measurement in superficial feet as plates, we select the multiplier for the particular thickness as given in the table, and the product is the weight in pounds.

To determine the weight of a piece of wrought iron, we ascertain its contents in cubic inches, and multiply it by the decimal .281; 'or in feet, and multiply by 486.8; or, if it admits of measurement as a plate, multiply the amount of superficial feet by the figures set against the particular thickness in the table. To determine the weight of any piece of round, square, or flat iron, we select the amount given in the table, and multiply it by the number of feet in length of the piece whose weight we wish to obtain.