The only machine of the series of portable riveters employed by Messrs M‘Millan which remains to be noticed is that which overtakes the riveting of keels. This machine is perhaps one of the most perfect of the series, performing its functions satisfactorily, viewed from whatever standpoint. The riveting required on the keel of large vessels is very heavy, especially if the through-keelson and side-bar system is adopted, when five thicknesses of plate have to be connected, the rivets employed being 1⅛-inch or 1¼-inch in diameter. The situation is not favourable for getting at the work to be done, the head-room available not often exceeding 2½ or 3 feet. These conditions render great compactness, together with portableness, necessary in the machine. The keel itself was utilised for the attachment of the Tweddell riveter as first tried, then again a sort of light trestle was employed, the riveter being at one end of a lever racking on this. These plans were abandoned, however, in favour of the machine as now used in various yards throughout the country, an illustration of which is given by Fig. 23. A low carriage is travelled down alongside the keel. This carriage supports a balanced lever, carrying at one end the riveter, capable of exerting about 50 tons on the rivet head, and at the other a balance weight. This lever can in its turn revolve horizontally about a short pillar fixed on a turn-table, thus affording unlimited control over the riveter by the man in charge; enabling him, indeed, to adjust the riveter to every irregularity of position or direction of the rivets in keel. As many as 420 1¼-inch rivets per day have been put in by this machine, an amount which is fully equal to the work of two squads of riveters, and in one yard 70 rivets have been closed in as many consecutive minutes. [Illustration: FIG. 23. TWEDDELL HYDRAULIC KEEL RIVETER.] It may be stated generally that the several hydraulic riveters require two men to work them, and the rivets are heated in portable furnaces and dealt out in any quantities required, by a boy in attendance. The quality of the work done is superior to hand work, chiefly in that when rivets are well heated the pressure is equalised, and affects the rivets throughout their entire length, filling the holes to their utmost. This advantage tells more in the case of keel riveting, and that it is so is evidenced by the fact, as communicated by a foreman having great experience, that rivets ¼-inch longer than rivets closed by hand have even less superfluous surface material when closed by the machine.