Compression is the power exerted on a post when loaded with a superincumbent weight, as that which is exerted on the collar-beam, or struts and rafters of a truss.

It has been discovered, that a timber, if placed as a post, whether long or short, would in either case, if entirely inflexible, support a weight equally well. But, inasmuch as there is some flexibility in all timber, a piece will, if higher than about seven times its diameter, bend and break before it can be crushed by compression; and it is stated, that a piece, if a hundred times as high as it is in diameter, will be incapable of supporting the smallest weight*

The nature of the subject under consideration makes it next to impossible to determine rules which will be of much service. As the compressive strength of timber is so variable in different specimens, and in none so geometrically proportionate to its length as to give reliablfe data, rules for ascertaining the exact size for all purposes and situations would only confuse, if not deceive, the mechanic.

* Gwilt's "Encyclopaedia of Architecture," p. 442, art. 1600.

The power of resistance to compression is so great, that no serious danger need be apprehended from the use of such dimensions of timber for collar-beams, truss-rafters, struts, etc. (these being, when in use, in a state of compression), as will generally agree with the tie-beams and purlins; and the only rule that may be considered of value is, that the compressed pieces of any work should bear such a proportion to those subjected to a tensile or cross strain as will make the whole truss, whatever its design, comparatively uniform in appearance throughout.

The following table, prepared by Mr. Tredgold, was designed to aid in determining the strength of timber when compressed in the direction of its length. The calculations were made for foreign timber. It is presumed, however, they are quite as reliable for timber grown in America as for that grown in England, since in neither is their truth susceptible of mathematical demonstration.

Kind of Wood.

Proportional Strength.

English Oak.......

.....0015

Beech.....

.....00195

Alder........

.....0023

Green Chestnut........

.....00267

Ash........

Elm......

.....00184

Locust......

.....00152

Rega Fir.........

.....00152

Memel Fir.........

.....00133

Norway Spruce........

.....00142

White Pine......

..........................00157

Larch..........

..........................0019

Problem VI. To Determine The Diameter Of A Round Column That Will Support A Given Weight

Rule. - Multiply the weight in pounds by 1.7 times the amount set against the kind of wood in the table; then multiply the square-root of the product by the length or height in feet; and the square-root of the last product will be the diameter of the post in inches.

If the column be shorter than ten times its diameter, the dimensions ascertained by this rule will be too small; in which case, the true diameter may be determined by Problem VIII.

Problem VII. To Determine The Size Of A Rectangular Post That Will Support A Given Weight

Rule. - Multiply the weight in pounds by the square of the length of the post in feet, and this product by the number set against the kind of wood in the table. Divide the product by the breadth in inches, and the cube-root of the quotient will be the thickness in inches.

In case the post is less than ten diameters high, the dimensions will be determined by Problem VIII., as before directed.