rate of diminution, is almost precisely one third of the total, or maximum strength of the iron at ordinary temperatures."

From the mean of all the rates in the above Table the following rule is deduced: "the thirteenth power of the temperature above 80° is proportionate to the fifth power of the diminution from the maximum tenacity."

Professor W. R. Johnson, a member of the sub-committee, has since applied the results developed in the preceding experiments to practical purposes, in increasing the tenacity of wrought iron by subjecting it to tension under a high degree of temperature, before using it for purposes in which it will have to undergo considerable strains, as, for example, in chain cables, &c.

This subject was brought by Prof. Johnson before the Board of Navy Commissioners in 1841; subsequently, experiments were made by him under direction of the Navy Department, the results of which, as exhibited in the following Table, were published in the Senate Public Documents, (1) 28th Congress, 2d Session, p. 641. Dec. 3, 1844.

Table of the effects of Thermo-tension on the Tenacity and Elongation of Wrought Iron.

Prof. Johnson in his letter remarks: "It will be observed that in these experiments the temperature has, with a view to economy of time, been limited to 400°, whereas the best effects of the process have generally been obtained heretofore when the heat has been as high as 575°."

332. Resistance of Iron Wire to Impact. The following Table of experiments gives the results obtained by Mr. Hodgkinson, by suspending an iron ball at the end of a wire, (diameter No. 17,) and letting another iron ball impinge upon it from different altitudes. The suspended and impinging balls had holes drilled through them, through which the wire passed. A disc of lead was placed on the suspended ball to receive the blow, and lessen the recoil from elasticity.

The following observations are made by Mr. Hodgkinson: "To ascertain the strength and extensibility of this wire, it was broken in a very careful experiment with 252 lbs., suspended at its lower end, and laid gradually on. And to obtain the increment of a portion of the wire (length 24 ft. 8 in.) when loaded by a certain weight, it had 139 lbs. hung at the bottom, and when 89 lbs. were taken off the load, the wire decreased in length .39 inch.

"Should it be suggested that the wire by being frequently impinged upon would perhaps be much weakened, the author would beg to refer to a paper of his on Chain Bridges, Manchester Memoirs, 2d series, vol. 5, where it is shown that an iron wire broken by pressure several times in succession is very little weakened, and will nearly bear the same weight as at first."

"The first of the preceding experiments on wires are the only ones from which the maximum can, with any approach to certainty, be inferred; and we see from them that the wire resisted the impulsion with the greatest effect when it was loaded at bottom with a weight, which, added to that of the striking body, was a little more than one third of the weight that would break the wire by pressure."

"From these experiments generally, it appears that the wire was weak to bear a blow when lightly loaded."

"These last experiments and remarks, and some of the preceding ones," (on horizontal impact,)" show clearly the benefit of giving considerable weight to elastic structures subject to impact and vibration."

333. Resistance to Torsion of Wrought and Cast Iron. The following Table exhibits the results of experiments made by Mr. Dunlop, at Glasgow, on round bars of wrought iron. The twisting weights were applied with an arm of lever 14 feet 2: inches.

Table of experiments made by Mr. G. Rennie upon Cast and Wrought Iron. Weight applied at an arm of lever of 2 feet.

334. Strength of Copper. The various uses to which copper is applied in constructions, render a knowledge of its resistance under various circumstances a matter of great interest to the engineer.

Resistance to Extension. The resistance of cast copper on the square inch, from the experiments of Mr. G. Rennie, is 8.51 tons, that of wrought copper reduced per hammer at 15.08 tons. Copper wire is stated to bear 27.30 tons on the square inch. From the experiments made under the direction of the Franklin Institute, already cited, the mean strength of rolled sheet copper is stated at 14.35 tons per square inch.

Resistance to Compression. Mr. Rennie's experiments on cubes of one fourth of an inch on the edge, give for the crushing

weight of a cube of cast copper 7318 lbs., and of wrought copper 6440 lbs.

335. Effects of Temperature on Tensile Strength. The experiments already cited of the Franklin Institute, show that the difference in strength at the lower temperatures, as between 60° and 90°, is scarcely greater than what arises from irregularities in the structure of the metal at ordinary temperatures. At 550° Fahr. copper loses one fourth of its tenacity at ordinary temperatures, at 817° precisely one half, and at 1000° two thirds.

Representing the results of experiments by a curve of which the ordinates represent the temperatures above 32o, and the abscissas the diminutions of tenacity arising from increase of temperature, the relations between the two will be thus expressed: the squares of the diminutions are as the cubes of the tempera

tures.

336. STRENGTH OF OTHER METALS. Mr. Rennie states the tenacity of cast tin at 2.11 tons per square inch; and the resistance to compression of a small cube of of an inch on an edge at 966 lbs.

In the same experiments, the tenacity of cast lead is stated at 0.81 tons per square inch; and the resistance of a small cube of same size as in preceding paragraph at 483 lbs.

In the same experiments, the tenacity of hard gun-metal is stated at 16.23 tons; that of fine yellow brass at 8.01 tons. The resistance to compression of a cube of brass the same as beforementioned, is stated at 10304 lbs.

337. Linear Dilatation of Metals by Heat. The following Table is taken from results of experiments on the dilatation of solids, by Professor Daniell, published in the Philosophical Transactions, 1831.

Table of Dimensions which a bar takes whose length at 62° is

338. Adhesion of Iron Spikes to Timber. The following Tables and results are taken from an article, by Professor Walter R. Johnson, published in the Journal of the Franklin Institute, vol. 19, 1837, giving the details of experiments made by him on spikes of various forms driven into different kinds of timber.

339. The first series of experiments was made with Burden's plain square spike, the flanched, grooved, and swell spike, and the grooved and swelled spike. The timber was seasoned Jersey yellow pine, and seasoned white oak.

From these experiments it results, that the grooved and swelled form is about 5 per cent. less advantageous than the plain, in yellow pine, and about 18 per cent. superior to the plain in oak. The advantage of seasoned oak over the seasoned pine, for retaining plain spikes, is as 1 to 1.9, and for grooved spikes as 1 to

2.37.

340. The second series of experiments, in which the timber was soaked in water after the spikes were driven, gave the following results.

For swelled and grooved spikes, the order of retentiveness was, 1 locust; 2 white oak; 3 hemlock; 4 unseasoned chesnut; 5 yellow pine.

For grooved spike without swell, the like order is—1 unseasoned chesnut; 2 yellow pine; 3 hemlock.

The swelled and grooved spike was, in all cases, found to be inferior to the same spike with the swell filed off.

341. The third series of experiments gave the following results. Thoroughly seasoned oak is twice, and thoroughly seasoned locust 23 times as retentive as unseasoned chesnut.

The forces required to extract spikes are more nearly proportional to the breadths than to either the thickness or the weights of the spikes. And, in some cases, a diminution of thickness with the same breadth of spike afforded a gain in retentiveness.

"In the softer and more spongy kinds of wood the fibres, instead of being forced back longitudinally and condensed upon themselves, are, by driving a thick, and especially a rather obtusely-pointed spike, folded in masses backward and downward so as to leave, in certain parts, the faces of the grain of the timber in contact with the surface of the metal."

"Hence it appears to be necessary, in order to obtain the greatest effect, that the fibres of the wood should press the faces as nearly as possible in their longitudinal direction, and with equal intensities throughout the whole length of the spike."

The following is the order of superiority of the spikes from that of the ratio of their weights and extracting forces respectively.