* It is a common and excellent practice amongst the Sheffield workmen to use coke both in forging and hardening steel goods. They frequently prepare it for themaelves, either upon the forge hearth or in a heap in the open yard. In that celebrated town a decided preference is given, for the purpose of hardening, to the light coke of the Deepcar coal, which is obtained about eight miles N.W. of Sheffield, although for ordinary use and for forging, this is considered an inferior kind of coal and of light quality: other workmen prefer charcoal for hardening.

Less than a certain heat fails to produce hardness, and in the opinion of some workmen has quite the opposite effect, and they consequently resort to it as the means of rapid annealing, not however, by plunging the steel into the water and allowing it to remain until cold; but dipping it quickly, holding it in the steam for a few moments, dipping it again and so on, reducing it to the cold state in a hasty but intermittent manner.

There is another opinion prevalent amongst workmen, that steel which is "pinny" or as if composed of a bundle of hard wires, is rendered uniform in its substance if it is first hardened and then annealed.

Secondly, the choice of the cooling medium has reference mainly to the relative powers of conducting heat they severally possess: the following have been at different times resorted to with various degrees of success: currents of cold air; immersion in water in various states, in oil or wax, and in freezing mixtures; mercury, and flat metallic surfaces have been also used. Mr. Stodart recommended as the result of his experiments, plain water at a temperature of 40° Fahrenheit. On the whole, however, there appears to be an opinion that mercury gives the greatest degree of hardness; then cold salt and water, or water mixed with various "astringent and acidifying matters;" plain water follows; and lastly, oily mixtures.*

* It is argued by some, that by heating pieces of steel to different degrees, before plunging them into the water, the one piece attains full hardness, the next the temper of a tool fit for metal, another of a tool fit for wood, a fourth that of a spring, and so on. That this view is not altogether without foundation, appears in the fact that if the end of a piece of steel be made entirely hard, the transition is not quite immediate from the hard to the soft part; and Mr. Ross, in making the dividing-points, for his dividing-engine, hardens the end of a longer piece of steel than is required, and forms the point upon the grindstone, exactly at the part where the temper suits, without the steel being let down at all; a practice first employed by Mr. Stancliffe, a workman formerly employed by the celebrated Ramsden. In hardening by this method however without tempering, the scale of proper hardness is confined within such extremely narrow limits, as to be nearly useless; thus, it frequently happens that in a number of tools heated as nearly alike as the workman could judge, some few will be found too soft for any use, although they were all intended to receive the ordinary hardness, so as to require letting down, as usual with those tools exposed to violent strains or blows, such as screw-taps, cold chisels and hatchets, although many tools for metal, used with quiet and uniform pressure, are left of the full hardness for greater durability,

With the access of heat, beyond the lowest that will suffice, the brittleness rather than the useful hardness of tools is increased; and when no excess of heat is employed beyond that absolutely requisite for hardening in the usual manner, the steel does not appear to be injured, and the colours on its brightened surface that occur in tempering are an excellent, and in general, sufficiently trustworthy index of the inferior degrees of hardness proper for various uses.

A so-called natural spring is made by a vessel with a true and a false bottom, the latter perforated with small holes; it is filled with water, and a copious supply is admitted beneath the partition; it ascends through the holes, and pursues the same current as the heated portions, which also escape at the top. This was invented by the late John Oldham, Esq., Engineer to the Bank of England, and was used by him in hardening the rollers for transferring the impressions to the steel-plates for bank-notes.

Sometimes when neighbouring parts of works are required to be respectively hard and soft, metal tubes or collars are fitted tight upon the work, to protect the parts to be kept soft from the direct action of the water, at any rate for so long a period as they retain the temperature suitable to hardening.

The process of hardening is generally one of anxiety, as the

* I find but one person who has commonly used the mercury; many presume upon the good conducting power of the metal, and the nonformation of steam, which causes a separation betwixt the steel and water when the latter is employed as the cooling medium. I have failed to learn the reason of the advantage of salt and water, unless the fluid hare, as well as a greater density, a superior conducting power. The file-makers medicate the water in other ways, but this is one of the questionable mysteries which is never divulged; although it is supposed that a small quantity of white asenic is generally added to water saturated with salt One thing however, may be noticed, that articles hardened in salt and water are apt to rust, unless they are laid for a time in lime-water, or some neutralising agent sudden transition from heat to cold often causes the works to become greatly distorted if not cracked. The last accident is much the most likely to occur with thick massive pieces, which are as it were hardened in layers, as although the external crust or shell may be perfectly hard, there is almost a certainty that towards the center the parts are gradually less hard; and when broken the inner portions will sometimes admit of being readily filed.

With plain water an opinion very largely exists in favour of that which has been used over and over again even for years, provided it is not greasy; and when the steel is very harsh, the chill is taken off plain water to lessen the risk of cracking it; oily mixtures impart to this articles, such as springs, a sufficient and milder degree of hardness, with less danger of cracking, than from water; and in some cases a medium course is panned by covering the water with a thick film of oil, which is said to be adopted occasionally with scythes, reaping-hooks, and thin edge-tools.

Having experimented upon all these means, I am induced fully to acquiesce in Mr. Stooart's recommendation of plain cold water for general purposes; except in a case of thin elastic works, for which oil, or oily compositions are certainly more proper, and some of these are described in page 249.

When in the fire the steel becomes altogether expanded, and in the water its outer crust is suddenly arrested, but with a tendency to contract from the loss of heat, which cannot so rapidly occur at the central part; it may be therefore presumed that the inner bulk continues to contract after the outer crust is fixed, and which tends to tear the two asunder, the more especially if there be any defective part in the steel itself. An external flake of greater or less extent not unfrequently shells off in hardening; and it often happens that works remain unbroken for hours after removal from the water, but eventually give way and crack with a loud report, from the rigid unequal tension produced by the violence of the process of hardening.

The contiguity of thick and thin parts is also highly dangerous, as they can neither receive, nor yield up heat, in the same times; the mischief is sometimes lessened by binding pieces of metal around the thin parts with wire, to save them from the action of the cooling medium. Sharp angular notches are also fertile sources of mischief, and where practicable they should be rejected in favour of curved lines.

As regards both cracks and distortions, it may perhaps be generally said, that their avoidance depends principally upon manipulation, or the successful management of every step: first the original manufacture of the steel, its being forged and wrought, so that it may be equally condensed on all sides with the hammer, otherwise when the cohesion of the mass is lessened from its becoming red-hot, it recovers in part from any unequal state of density in which it may have been placed.

Whilst red-hot, it is also in its weakest condition; it is therefore prone to injury either from incautious handling with the tongs, or from meeting the sudden cooling action irregularly, and therefore it is generally best to plunge works vertically, as all parts are then exposed to equal circumstances, and less disturbance is risked than when the objects are immersed obliquely or sideways into the water; although lor swords, and objects of similar form, it is found the best to dip them exactly as in making a vertical downward cut with a sabre, which for this weapon is its strongest direction. Occasionally objects are clamped between stubborn pieces of metal, as soft iron or copper, during their passage through the fire and water. Such plans can be seldom adopted and are rarely followed, the success of the process being mostly allowed to depend exclusively upon good general management.* In all cases the thick unequal scale left from the forge should be ground off before hardening, in order to expose a clean metallic surface, otherwise the cooling medium cannot produce its due and equal effect throughout the instrument. The edges also should be left thick, that they may not be burned in the fire; thus it will frequently happen that the extreme end or edge of a tool is inferior in quality to the part within, and that the instrument is much better after it has been a few times ground; in uring this point, I cannot do better than quote the couplet inserted in Moxon's Mechanick Exercises, and which he describes as having been very old and familiar to smiths even in his day -