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5th. It is of the greatest importance that the stones or bricks, with which Portland cement is used, should be thoroughly soaked with water. If under water, in a quiescent state, the cement will be stronger than out of water.

6th. Blocks of brickwork, or of concrete, made with Portland cement, if kept under water until required for use, would be much stronger than if kept dry.

7th. Salt water is as good for mixing with Portland cement as fresh water.

8th. Roman cement is very ill adapted for being mixed with sand.

9th. The resistance of a block of pure Portland cement to extension after an immersion of one year was about 480 lbs. on the square inch; whilst the resistances of blocks made of sand and cement, after the same period of immersion, decreased with the quantity of sand added. Blocks of one volume of cement in paste to one of sand giving three-fourths the resistance of those of pure cement; and those of one volume of cement to five of sand giving only one-sixth of the resistance of blocks of pure cement.

10th. Roman cement is only one-third the strength of Portland cement.-Proceedings of the Institution of Civil Engineers, Vol. XXV., p. 66.

342. Concrete and Beton. From experiments made on concrete, prepared according to the most approved process in England, by Colonel Pasley, it appears that this material is very inferior in strength to good brick, and the weaker kinds of natural stones.

From experiments made by Colonel Totten on béton, the following conclusions are drawn:

That béton made of a mortar composed of hydraulic cement, common lime, and sand, or of a mortar of hydraulic cement and sand, without lime, was the stronger as the quantity of sand was the smaller. But there may be 0.50 of sand, and 0.25 of common lime, without sensible deterioration; and as much as 1.00 of sand, and 0.25 of lime, without great loss of strength.

Béton made with just sufficient mortar to fill the void spaces between the fragments of stone was found to be less strong than that made with double this bulk of mortar. But Colonel Totten remarks, that this result is perhaps attributable to the difficulty of causing so small a quantity of mortar to penetrate the voids, and unite all the fragments perfectly, in experiments made on a small scale.

The strongest béton was obtained by using quite small

fragments of brick, and the weakest from small, rounded, stone gravel.

A béton formed by pouring grout among fragments of stone, or brick, was inferior in strength to that made in the usual way with mortar.

Comparing the strength of the bétons on which the experiments were made, which were eight months old when tried, with that of a sample of sound red sandstone of good quality, it appears that the strongest prisms of béton were only half as strong as the sandstone.

The working strain on masses of concrete, brick, and rubble masonry seldom exceeds in structures that of one-sixth of the crushing weight of blocks of these materials.

IV.

STRENGTH OF TIMBER.

A wide range of experiments has been made on the resistance of timber to compression, extension, and a transverse strain, presenting very variable results. Among the most recent, and which command the greatest confidence from the ability of their authors, are those of Professor Barlow and Mr. Hodgkinson: the former on the resistance to extension and a transverse strain; the latter on that to compression.

The following Table, taken from Vol. V. Professional Papers of the English Royal Engineers. No. V. Remarks and Experiments on various Woods, give some valuable results on American timber subjected to a strain parallel to the fibre. The column marked C gives the cohesive strength.

343. Resistance to Tensile Strain. The following table exhibits the specific gravity, and the mean resistance per square inch of various kinds of timber, from the experiments of Prof. Barlow.

The working strain on beams subjected to extension should not exceed of the rupturing strain in permanent structures, but for temporary purposes, like scaffolding, &c., it may be placed at 4th the rupturing strain with safety.

But few direct experiments have been made upon the elongations of timber from a strain in the direction of the fibres. From some made in France by MM. Minard and Desormes, it would appear that bars of oak having a sectional area of one square inch will be elongated .001176 of their length by a strain of one ton.

344. Resistance to Compressive Strains. The following Table exhibits the results obtained by Mr. Hodgkinson from experiments on short cylinders of timber with flat ends. The diameter of each cylinder was one inch, and its height two inches. The results, in the first column, are a mean from about three experiments on timber moderately dry, being such as is used for making models for castings; those in the second column were obtained in a like manner, from similar