The proportions of the above compositions are given for 100 parts, by weight, with the exception of lacker 2.

The beautiful black polish on the Berlin castings for ornamental purposes, is said to be produced by laying the following composition on the hot iron, and then baking it.

Enough oil of turpentine is to be added to this mixture to make it spread.

266. From experiments made by Mr. Mallet, on the preservative properties of paints and varnishes for iron immersed in water, it appears that caoutchouc varnish is the best for iron in hot water, and asphaltum varnish under all other circumstances; but that boiled coal-tar, laid on hot iron, forms a superior coating to either of the foregoing.

267. Mr. Mallet recommends the following compositions for a paint, termed by him zoofagous paint, and a varnish to be used to preserve zincked iron both from corrosion and from fouling in

sea water.

Varnish for zincked Iron.

To 50 lbs. of foreign asphaltum, melted and boiled in an iron vessel for three or four hours, add 16 lbs. of red lead and litharge ground to a fine powder, in equal proportions, with 10 gals. of

drying linseed oil, and bring the whole to a nearly boiling temperature. Melt, in a second vessel, 8 lbs. of gum-animé; to which add 2 gals. of drying linseed oil at a boiling heat, with 12 lbs. of caoutchouc partially dissolved in coal-tar naphtha. Pour the contents of the second vessel into the first, and boil the whole gently, until the varnish, when taken up between two spatulas, is found to be tough and ropy. This composition, when quite cold, is to be thinned down for use with from 30 to 35 gals. of spirits of turpentine, or of coal naphtha.

268. It is recommended that the iron should be heated before receiving this varnish, and that it should be applied with a spatula, or a flexible slip of horn, instead of the ordinary brush.

When dry and hard, it is stated that this varnish is not acted upon by any moderately diluted acid, or alkali; and, by long immersion in water, it does not form a partially soluble hydrate, as is the case with purely resinous varnishes and oil paints. It can with difficulty be removed by a sharp-pointed tool; and is so elastic, that a plate of iron covered with it may be bent several times before it will become detached.

Zoofagous Paint.

269. To 100 lbs. of a mixture of drying linseed oil, red lead, sulphate of barytes, and a little spirits of turpentine, add 20 lbs. of the oxychloride of copper, and 3 lbs. of yellow soap and common rosin, in equal proportions, with a little water.

When zincked iron is exposed to the atmosphere alone, the varnish is a sufficient protection for it; but when it is immersed in sea water, and it is desirable, as in iron ships, to prevent it from fouling, by marine plants and animals attaching themselves to it; the paint should be used, on account of its poisonous qualities. The paint is applied over the varnish, and is allowed to harden three or four days before immersion.

RESULTS OF EXPERIMENTAL RESEARCHES ON THE
STRENGTH OF MATERIALS.

270. Whatever may be the physical structure of materials, whether fibrous or granular, experiment has shown that they all possess certain general properties, among the most important of which to the engineer are those of contraction, elongation, deflection, torsion, and lateral adhesion, and the resistances which these offer to the forces by which they are called into action.

271. All solid bodies, when submitted to strains by which any of these properties are developed, have, within certain limits, termed the limits of elasticity, the property of wholly or partially resuming their original state, when the strain is taken off. This property is usually denominated the elastic force, and has for its measure, in the case of contraction, or elongation, the ratio between the force which causes the one or the other of these states and the fraction which measures the degree of contraction, or elongation.

272. To what extent bodies possess the property of total recovery of form, when relieved from a strain, is still a matter of doubt. It has been generally assumed, that the elasticity of a material does not undergo permanent injury by any strain less than about one third of that which would entirely destroy its force of cohesion, thereby causing rupture. But from the most recent experiments on this point made by Mr. Hodgkinson on cast iron, it appears that the restoring power of this material is destroyed by very slight strains; and it is rendered probable that this and most other materials receive a permanent change of form, or set, under any strain, however small.

273. The extension, or contraction of a solid, may be effected either by a force acting in the direction in which the contraction, or elongation takes place, or by one acting transversely, so as to bend the body. Experiments have been made to ascertain, directly, the proportion between the amount of contraction, or elongation, and the forces by which they are produced. From these experiments, it results, that the contractions, or elongations are, within certain limits, proportional to the forces, but that an equal amount of contraction, or elongation, is not produced by the same amount of force. From the experiments of Mr. Hodgkinson and M. Duleau, it appears, that in cast and malleable iron the contraction, or elongation, caused by the same amount of pressure, or tension, is nearly equal; while in timber, according to Mr. Hodgkinson, the amount of contraction is about four fifths of the elongation for the same force.

274. When a solid of any of the materials used in constructions is acted upon by a force so as to produce deflection, experiment has shown that the fibres towards the concave side of the bent solid are contracted, while those towards the convex side are elongated; and that, between the fibres which are contracted and those which are elongated, others are found which have not undergone any change of length. The part of the solid occupied by these last fibres has received the name of the neutral line, or neutral axis.

275. The hypothesis usually adopted, with respect to the circumstances attending this kind of strain, is that the contractions

and elongations of the fibres on each side of the neutral axis are proportional to their distances from this line; and that, for slight deflections, the neutral axis passes through the centre of gravity of the sectional area. From experiments, however, by Mr. Hodgkinson and Mr. Barlow, it appears that the neutral axis, in forged iron and cast iron, lies nearer to the concave than to the convex surface of the bent solid, and, probably, shifts its position when the degree of deflection is so great as to cause rupture. In timber, according to Mr. Barlow, the neutral axis lies nearest to the convex surface; and, from his experiments on solids of forged iron and timber with a rectangular sectional figure, he places the neutral axis at about three eighths of the depth of the section from the convex side in timber, and between one third and one fifth of the depth of the section from the concave side in forged iron.

276. When the strain to which a solid is subjected is sufficiently great to destroy the cohesion between its particles, and cause rupture, experiment has shown that the force producing this effect, whether it act by tension, so as to draw the fibres asunder, or by compression, to crush them, is proportional to the sectional area of the solid. The measure, therefore, of the resistance offered by a solid to rupture, in either of these cases, is that force which will rupture a sectional area of the solid represented by unity.

277. From experiments made to ascertain the circumstances. of rupture by a tensile force, it appears that the solid torn apart exhibits a surface of fracture more or less even, according to the nature of the material.

278. Most of the experiments on the resistance to rupture by compression, have been made on small cubical blocks, and have given a measure of this resistance greater than can be depended upon in practical applications, when the height of the solid exceeds three times the radius of its base. This point has been very fully elucidated in the experiments of Mr. Hodgkinson upon the rupture by compression of solids with circular and rectangular bases. These experiments go to prove, that the circumstances of rupture, and the resistance offered by the solid, vary in a constant manner with its height, the base remaining the same. In columns of cast iron, with circular sectional areas, it was found that the resistance remained constant for a height less than three times the radius of the base; that, from this height to one equal to six times the radius of the base, the resistance still remained constant, but was less than in the former case; and that, for any height greater than six times the radius of the base, the resistance decreased with the height. In the two first cases, the solids were found to yield either by the upper portion sliding off 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 resistances, 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.

279. 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 resistances offered within these limits are proportional to the areas of the bases.

280. 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.

281. STRENGTH OF STONE. 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.