2d. That common mortar is the stronger and harder as the quantity of sand is less.

3d. That any addition of common lime to a mortar of hydraulic cement and sand weakens the mortar, but that a little lime may be added without any considerable diminution of the strength of the mortar, and with a saving of expense.

4th. The strength of common mortars is considerably improved by the addition of an artificial puzzolana, but more so by the addition of an hydraulic cement.

5th. Fine sand generally gives a stronger mortar than coarse sand.

6th. Lime slaked by sprinkling gave better results than lime slaked by drowning. A few experiments made on air-slaked lime were unfavorable to that mode of slaking.

7th. Both hydraulic and common mortar yielded better results when made with a small quantity of water than when made thin. 8th. Mortar made in the mortar-mill was found to be superior to that mixed in the usual way with a hoe.

9th. Fresh water gave better results than salt water.

295. STRENGTH OF 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 beton, the following conclusions are drawn:

That beton 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.

Beton 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 beton was obtained by using quite small fragments of brick, and the weakest from small, rounded, stone gravel. A beton 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 betons on which the experiments were made, which were eight months old when tried, with

that of a sample of sound red sand-stone of good quality, it ap pears that the strongest prisms of beton were only half as strong as the sand-stone.

296. 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.

297. Resistance to Extension. 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.

298. 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.

299. Resistance to Compression. 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 specimens, which were turned and kept dry in a warm place two months longer. A comparison of the results in the two columns, shows the effect of drying on the strength of timber; wet timber not having half the strength of the same when dry. The circumstances of rupture were the same as already stated in the general observations under this head; the height of the wedge which would slide off in timber being about half the diameter, or thickness of the specimen crushed.

made a number of invaluable experiments on the strength of pillars of timber, and of columns of iron and steel, and from them has deduced formula for calculating the pressure which they will support before breaking. The experiments on timber were made on pillars with flat ends. The following are the formulæ from which their strength may be estimated.

Calling the breaking weight in lbs. W.

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the side of the square base in inches d.

the length of the pillar in feet l.

Then for long columns of oak, in which the side of the square base is less than 7th the height of the column;

For shorter pillars, where the ratio between their thickness and height is such that they still yield by bending, the strength is estimated by the following formula:

Calling the weight calculated from either of the preceding formulæ, W.

Calling the crushing weight, as estimated from the preceding Table, W'.

Calling the breaking weight in lbs., W".

Then the formula for the strength is

In each of the preceding formulæ d must be taken in inches, and 7 in fect.

301. Resistance to Transverse Strains. As timber, from the purposes to which it is applied, is for the most part exposed to a transverse strain, the far greater number of experiments have been made to ascertain the relations between the strain, the deflection caused by it, and the linear dimensions of the piece subjected to the strain. These relations have been made the subject of mathematical investigations, founded upon data derived from experiment, which will be given in the APPENDIX. The following Table exhibits the results of experiments made upon beams having a rectangular sectional area, which were laid horizontally upon supports at their ends, and subjected to a strain applied at the middle point between the supports, in a vertical direction.

For a more convenient application of the formula to the results of the experiments, the notation adopted in the preceding Art. will be here given.

Call the transverse force necessary to break the beam in lbs., W. the distance between the supports in inches, l.

"6 the horizontal breadth of the sectional area in inches, b. "the vertical depth

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the deflection arising from a weight w in inches, f.

Table of Experiments with the foregoing Notation.

d.

302. Resistance to Detrusion. From the experiments of Prof. Barlow, it appears that the resistance offered by the lateral adhesion of the fibres of fir, to a force acting in a direction parallel to the fibres, may be estimated at 592 lbs. per square inch.

Mr. Tredgold gives the following as the results of experiments on the resistance offered by adhesion to a force applied perpendicularly to the fibres to tear them asunder.

303. STRENGTH OF CAST IRON. The most recent experiments on the strength of this material are those of Mr. Hodgkinson. Those, particularly, made by him on the subject of the strength of columns, and the most suitable form of cast-iron beams to sustain a transversal strain, have supplied the engineer and architect with the most valuable guide in adapting this material to the various purposes of structures.

304. Resistance to Extension. From a few experiments made by Mr. Rennie and Captain Brown, the tensile strength of cast iron varies from 7 to 9 tons per square inch.

The experiments of Mr. Hodgkinson upon both hot and cold blast iron give the tensile strength from 6 to 93 tons per square inch.

From some experiments made on American cast iron, under the direction of the Franklin Institute, the mean tensile strength is 20834 lbs., or 91 tons per square inch.

305. Resistance to Compression. The general circumstances attending the rupture of this material by compression, drawn from