upon the lower, in the direction of a plane making a constant angle with the axis of the solid; or else by separating into conical or wedge-shaped blocks, having the upper and lower surfaces of the solid as their bases, the angle at the apex being double that made by the plane and axis of the solid. With regard to the resistance, it was found that they varied in the ratio of the area of the bases of the solids. Where the height of the solid was greater than six times the radius of the base, rupture generally took place by bending.

325. From experiments by Mr. Hodgkinson, on wood and other substances, it would appear that like circumstances accompany the rupture of all materials by compression; that is, within certain limits, they all yield by an oblique surface of fracture, the angle of which with the axis of the solid is constant for the same material; and that the resistance offered within these limits are proportional to the areas of the bases.

326. Among the most interesting deductions drawn by Mr. Hodgkinson, from the wide range of his experiments upon the strength of materials, is the one which points to the existence of a constant relation between the resistances offered by materials of the same kind to rupture from compression, tension, and a transverse strain. The following Table gives these relations, assuming the measure of the crushing force at 1000.

II.

STRENGTH OF STONE.

327. The marked difference in the structure and in the proportions of the component elements frequently observed in stone from the same quarry would lead to the conclusion that corresponding variations would be found in the strength of stones belonging to the same class, a conclusion which experiment has confirmed. The experiments made by different individuals on this subject, from not having been conducted in the same manner, and from the omission in most cases of details respecting the structure and component elements of the material tried, have, in some instances, led to contradictory results. A few facts, however, of a general character, have been ascertained, which may serve as guides in ordinary cases; but in important structures, where heavy pressures are to be sustained, direct experiment is the only safe course for the engineer to follow, in selecting a material from untried quarries.

328. Owing to the ease with which stones generally break under a percussive force, and from the comparatively slight resistance they offer to a tensile force and to a transverse strain, they are seldom submitted in structures to any other strain than one of compression; and cases but rarely occur where this strain is not greatly beneath that which the better class of building stones can sustain permanently, without undergoing any change in their physical properties. Where the durability of the stone, therefore, is well ascertained, it may be safely used without a resort to any specific experiment upon its strength, whenever, in its structure and general appearance, it resembles a material of the same class known to be good.

329. The following table exhibits the principal results of experiments made by Mr. G. Rennie, and published in the Philosophical Transactions of 1818. The stones tried were in small cubes, measuring one and a half inches on the edge. The table gives the pressure, in tons, borne by each superficial inch of the stone at the moment of crushing.

The following results are taken from a series of experiments made under the direction of Messrs. Bramah & Sons, and published in Vol. 1, Transactions of the Institution of Civil Engineers. The first column of numbers gives the weights, in tons, borne by each superficial inch when the stones commenced to fracture; the second column gives the crushing weight, in tons, on the same surface.

The following table is taken from one published in Vol.

2, Civil Engineer and Architect's Journal, which forms a part of the Report on the subject of selecting stone for the New Houses of Parliament. The specimens submitted to experiment were cubical blocks measuring two inches on an edge.

The numbers of the first column give the specific gravities; those in the second column the weight in tons on a square inch, when the stone commenced to fracture; and those in the third the crushing weight on a square inch.

The following table exhibits the results of experiments on the resistance of stone to a transverse strain, made by Colonel Pasley, on prisms 4 inches long, the cross section being a square of 2 inches on a side; the distance between the points of support 3 inches.

330. The conductors of the experiments on the stone for the New Houses of Parliament, Messrs. Daniell and Wheatstone, who also made a chemical analysis of the stones, and applied to them Brard's process for testing their resistance to

frost, have appended the following conclusions from their experiments:"If the stones be divided into classes, according to their chemical composition, it will be found that in all stones of the same class there exists generally a close relation between their various physical qualities. Thus it will be observed that the specimen which has the greatest specific gravity possesses the greatest cohesive strength, absorbs the least quantity of water, and disintegrates the least by the process which imitates the effects of weather. A comparison of all the experiments shows this to be the general rule, though it is liable to individual exceptions."

"But this will not enable us to compare stones of different classes together. The sandstones absorb the least quantity of water, but they disintegrate more than the magnesian limestones, which, considering their compactness, absorb a great deal."

331. Like conclusions to the preceding were reached by a commission for testing the properties of some of the stones and marbles used in the construction of the Capitol at Washington.

But few experiments have been made upon the strength and other properties of the building stones of the United States, and those of a local character. From the reports of a public commission, and of Professor R. Johnson, on the marbles and micaceous stratified stones used in the walls and foundations of the Capitol at Washington, the same general conclusions were arrived at as in the report of Messrs. Daniell and Wheatstone above cited. The strength of the marbles submitted to experiment varied from about seven thousand to twenty-four thousand pounds to the square inch; the micaceous stones used in the foundations averaged about fifteen thousand pounds to the square inch; some specimens of sandstone gave about five thousand pounds to the square inch; and