The following table exhibits the results of experiments made to test the resistance to compression of such woods as are in common use in this country for the purposes of construction.

Table: Resistance to Compression

Material.

Specific Gravity.

C.

H.

To crush fibres transversely 1/20 inch deep.

P.

To crush fibres longitudinally.

To separate fibres by sliding.

Value of P in Rules. Sensible Impression.

Pounds per inch.

Pounds per inch.

Pounds per inch.

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

0.613

9500

840

2250

900

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

0.762

11700

1160

2800

1120

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

0.774

8000

1250

2650

I060

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

0.369

7850

540

650

260

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

0.388

6650

480

800

320

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

0.423

5700

370

800

320

White Wood.................

0.397

3400

...

800

320

Chestnut..............................................................

0.491

6700

...

1250

500

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

0.517

5850

...

3000

I240

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

0.574

8450

...

2700

1080

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

0.877

13750

...

4100

1640

Cherry.................................................................

0.494

9050

...

2500

1000

Black Walnut.......................................................

0.421

7800

...

2100

840

Mahogany (St. Domingo)....................................

0.837

11600

...

5700

2280

" (Bay Wood)............................................

0.439

4900

...

1700

680

Live Oak..............................................................

0.916

11100

...

6800

2720

Lignum Vitae.....................................................

1.282

12100

...

7700

3080

The resistance of timber of the same name varies much; depending as it obviously must on the soil in which it grew on its age before and after cutting, on the time of year when cut, and on the manner in which it has been kept since it was cut. And of wood from the same tree much depends upon its location, whether at the butt or towards the limbs, and whether at the heart or of the sap, or at a point midway from the centre to the circumference of the tree. The pieces submitted to experiment were of ordinary good quality, such as would be deemed proper to be used in framing. The prisms crushed were generally small, about 2 inches long, and from I inch to 1 1/2 inches square; some were wider one way than the other, but all containing in area of cross section from I to 2 inches. The weight given in the table is the average weight per superficial inch.

Of the first six woods named, there were nine specimens of each tested; of the others, generally three specimens.

The results for the first six woods named are taken from the author's work on Transverse Strains, published by John Wiley & Sons, New York. The results for these six woods, as well as those for all the others named in the table, were obtained by experiments carefully made by the author. The first six woods named were tested in 1874 and 1876, and upon a testing machine, in which the power is transmitted to the pieces tested, by levers acting upon knife-edges. For a description of this machine, see Transverse Strains, Art. 704. The woods named in the table, other than the first six, were tested some twenty years since, and upon a hydraulic press, which, owing to friction, gave results too low.

The results, as thus ascertained, were given to the public in the 7th edition of this work, in 1857. In the present edition, the figures in Table I., for these woods, are those which have resulted by adding to the results given by the hydraulic press a certain quantity thought to be requisite to compensate for the loss by friction. Thus corrected, the figures in the table may be taken as sufficiently near approximations for use in the rules, - although not so trustworthy as the results given for the first six woods named, as these were obtained upon a superior testing machine, as above stated.

In the preceding table, the second column contains the specific gravity of the several kinds of wood, showing their comparative density. The weight in pounds of a cubic foot of any kind of wood or other material is equal to its specific gravity multiplied by 62.5, this number being the weight in pounds of a cubic foot of water. The third column contains the weight in pounds required to crush a prism having a base of one inch square; the pressure applied to the fibres longitudinally. In practice, it is usual never to load material exposed to compression with more than one fourth of the crushing weight, and generally with from one sixth to one tenth only. The fourth column contains the weight in pounds which, applied in line with the length of the fibres, is required to force off a part of the piece, causing the fibres to separate by sliding, the surface separated being one inch square. The fifth column contains the weight in pounds required to crush the piece when the pressure is applied to the fibres transversely, the piece being one inch thick, and the surface crushed being one inch square, and depressed one twentieth of an inch deep. The sixth column contains the value of P in the rules; P being the weight in pounds, applied to the fibres transversely, which is required to make a sensible impression one inch square on the side of the piece, this being the greatest weight that would be proper for a post to be loaded with per inch surface of bearing, resting on the side of the kind of wood set opposite in the table. A greater weight would, in proportion to the excess, crush the side of the wood under the post, and proportionably derange the framing, if not cause a total failure. It will be observed that the measure of this resistance is useful in limiting the load on a post according to the kind of material contained, not in the post, but in the timber upon which the post presses.