The resistance of materials to the force of stretching, as exemplified in the case of a rope from which a weight is suspended, is termed the resistance to tension. In fibrous materials, this force will be different in the same specimen, in accordance with the direction in which the force acts, whether in the direction of the length of the fibres or at right angles to the direction of their length. It has been found that, in hard woods, the resistance in the former direction is about eight to ten times what it is in the latter; and in soft woods, straight, grained, such as white pine, the resistance is from sixteen to twenty times. A knowledge of the resistance in the direction of the fibres is the most useful in practice.

In the following table are recorded the results of experiments made to test this resistance in some of the woods in common use, and also in iron, cast and wrought. Each specimen of the woods was turned cylindrical, and about 2 inches diameter, and then the middle part reduced to about 3/8 of an inch diameter, at the middle of the reduced part, and thence gradually increased toward each end, where it was considerably larger at its junction with the enlarged end. The results, in the case of the iron and of the first six woods named, are taken from the author's work, Transverse Strains, Table XX. Experiments were made upon the other three woods named by a hydraulic press, some twenty years since, and the results were first published in the 7th edition of this work, in 1857. These results, owing to friction, were too low. Adding to them what is supposed to be the loss by friction of the machine, the results thus corrected are what are given for these three woods in the following table, and may be taken as fair approximations, but are not so trustworthy as the figures given for the other six woods and for the metals.

Table II. - Resistance to Tension

Material.

Specific Gravity.

T.

Pounds required to rupture one inch square.

Georgia Pine................

0.65

16000

Locust.....................

0.794

24800

White Oak....................................

0. 774

19500

Spruce.............

0 432

19500

White Pine........................

0.458

12000

Hemlock.....................

0.402

8700

Hickory......................

0.751

26000

Maple..................

0.694

20000

Ash...........................................

0.608

15000

Cast Iron, American........ from

6.944

27000

,, English......... to

7.584

17000

Wrought Iron, American....from

7 600

60000

,, English...... to

7.792

50000

The figures in the table denote the ultimate capability of a bar one inch square, or the weight in pounds required to produce rupture. Just what portion of this should be taken as the safe capability will depend upon the nature of the strain to which the material is to be exposed. In practice it is found that, through defects in workmanship, the attachments may be so made as to cause the strain to act along one side of the piece, instead of through its axis; and that in this case fracture will be produced with one third of the strain that can be sustained through the axis. Due to this and other contingencies, it is usual to subject materials exposed to tensile strain with only from one sixth to one tenth of the breaking weight.