The length of these shafts is sometimes twenty-five times the length of their diameters, so that it is convenient to make them of several pieces. Two or three pieces may constitute the intermediate, and be of the finished forged diameter, thus avoiding drawing down ; but the two disc-ends should be made of lumps that are large enough to become the discs without upsetting.

Such shafts may be made, also, by welding the two discs to the shaft-ends. In these cases, the discs are made of fiat cakes, having holes punched in their middles and drifted to the diameter of the shaft. From the hole to one edge a piece is cut out previous to placing the disc upon the shaft, that the opening may be closed at the time of welding. The holes, in the discs are conical, and the larger part of the hole is outwards when on the shaft; and during the welding the shaft-end is sufficiently upset to fill the hole, by which the disc is riveted to the shaft. The straight piece of iron selected for the shaft itself is of the required forged diameter; all drawing down of this portion is therefore avoided.

But it is frequently advisable to make such an intermediate shaft of only one piece; the sectional area of the piece selected being equal to, or greater than, the sectional area of the intended disc. If the lump is greater in diameter than the disc required, the proper amount of reducing is given until the diameter of the disc is attained. After which, the necessary length of iron selected to produce the required length of intermediate, is ascertained by applying the appropriate rule in this form:

As the mean sectional area of the lump, is to the mean sectional area of the intermediate part of the shaft; so is the length of the intermediate required, to the necessary length of the lump to be reduced.

If the smith desires to make a shaft whose length of intermediate is 4500 millimetres, and whose forged diameter is 180 millimetres, he may select a lump whose diameter is 450 millimetres. Omitting small fractions, the sectional area of the lump to be operated upon is therefore 159043 square millimetres, and the first term of the proposition. The second term is represented by 25447, being the number of square millimetres in a sectional area of the intended intermediate. The length of this portion, 4500 millimetres, becomes the third term; and the fourth term that indicates the requisite length of iron is seen in the complete proposition thus:

159043 : 25447 :: 4500 : 720.

To this length of 720 millimetres for the intermediate, the thickness of the two discs is to be added. If the thickness of each intended disc is 90 millimetres, 180 millimetres are added to 720; and their sum denotes the total length of bar or lump to make one complete shaft, to be 900 millimetres. To this length 100 millimetres should be added for that which will be burnt or wasted during the several heatings. The precise amount requisite for the burnt portion principally depends upon the number of times the work is heated ; and this again depends upon the length of iron heated at one heat, and also upon the capability and power of the particular steam-hammer employed at the time.

Intel-mediate shafts and other screw-shafts of great length need considerable care in straightening. For this purpose, a large surface-table should be provided in the smithy, that the shaft may be laid upon it and straightened with a half-round top tool. In addition to the table, a thick, smooth cord should be used. The mode of applying the cord consists in making a straightedge of it by stretching it along the shaft-sides. A man may stand at each end of the shaft and stretch the cord while it is at an equal distance from the shaft at both ends; the cord being as near to the work as the heat will allow, but always applied to the sides of the work, and never above or beneath it. Another man may then observe any irregularities along the shaft by comparing them with the cord straight-edge,, after which the work is rectified with hammer and top-tool. Such straightening processes as these may be resorted to when it is not convenient to put the work into a lathe.