Such joints are applied to what are named hollow plungers, or trunk-plungers, and consist of two principal parts - the connecting-rod and the joint-head bolt. At the end of this bolt is a screw for a gun-metal nut, by which the bolt is secured to the plunger.

By referring to Fig. 42, it will be observed that the boss B of the connecting-rod is about equal in dimensions to the rectangular portion or boss at the other end ; and the smith will thus perceive that the shortest and most economical method of forging is by steam-hammering a bar to the dimensions of the two ends of the connecting-rod, and then by reducing the intermediate part till the desired length is attained. And to do this in a scientific manner instead of by a series of random hammerings and cutting pieces off, he will learn precisely how much metal is necessary to produce the rod to the proper length by applying the rule stated in page 8.

When the bar is reduced by sufficient welding and hammering to the dimensions of the two larger portions of the rod, the sectional area may be stated, the rule applied, and the length of bar necessary will be ascertained. If the sectional area of the lump which is being forged is 121/2 inches, and the required sectional area of the intermediate to be 7 inches, and the required length of the intermediate between the two bosses to be 20 inches, the proposition appears thus :

121 : 7 :: 20 : 11 3/16, 11.2 is the true amount. And although this is but little more than half the length of 20 inches, the smith may drive in a fuller at each extremity of the indicated distance, allowing only an eighth or a quarter of an inch for heating and burning the iron, and also for the metal being closed into a smaller space by hammering.

The forging of the joint-head bolt which is attached to the connecting-rod consists in making a bolt with a large solid head, the fibres in which are circularly arranged. This arrangement is obtained by heating a thick end to welding heat and placing it into a large half-round bottom-tool, which is on the floor if the work is too long to be stood lengthways upon an anvil. The cold end of the work is then struck by hammers until sufficiently upset, after which, the stem or screw end of the bolt is produced, either while the bolt is attached to the bar, or after being cut from the bar and held by the bolt-head.

The amount of iron necessary to produce the stem to its proper length is ascertained by the same method as for the connecting-rod, and the length of bar may then be cut off; but it is generally more convenient to reduce the bolt-stem to the diameter desired previous to cutting it from the bar.