The longest or back plate being cut to the proper length, is hammered down slightly at the extremities, and then curled round a mandrel the size of the suspension bolt. The side of the plate which is to fit against the others is then hollowed out by hammering; this is called “middling.” The next plate is then cut rather shorter than the first; the ends are tapered down so as not to disturb the harmony of the curve. This plate is middled on both sides. A slit is then cut at each end about ¾ of an inch in length and ⅜ inch wide, in which a rivet head slides to connect it with the first plate, so that in whatever direction the force acts these two plates sustain each other. At a little distance from this rivet a stud is formed upon the under surface by a punch, which forces out a protuberance which slides in a slit in the next plate. The next plate goes through precisely the same operations, except that it is 3 or 4 inches shorter at each end, and so on with as many plates as the spring is to consist of. The last plate, like the first, is of course only middled on one side. The plates of which the spring is to be composed having thus been prepared, have next to undergo the process of “hardening” and “tempering.” This is a very important branch of the business, and will bear a detailed description. There is no kind of tempering which requires so much care in manipulation as that of springs. It is necessary that the plates be carefully forged, not over-heated, and not hammered too cold; one is equally detrimental with the other. To guard against a plate warping in tempering, it is requisite that both sides of the forging shall be equally well wrought upon with the hammer; if not, the plates will warp and twist by reason of the compression on one side being greater than on the other.[1] The forge should be perfectly clean, and a good clean charcoal fire should be used. Or if coal be used it must be burned to coke in order to get rid of the sulphur, which would destroy the “life” of the steel. Carefully insert the steel in the fire, and slowly heat it evenly throughout its entire length; when the colour shows a light red, plunge it into lukewarm water—cold water chills the outer surface too rapidly—and let it lie in the water a short time. Animal oil is better than water; either whale or lard oil is the best, or lard can be used with advantage. The advantage of using oil is that it does not chill the steel so suddenly, and there is less liability to crack it. This process is called “hardening.”