reduction in thickness of shell plates below that due to an ordinary double-riveted joint, though we venture to submit that the triple-riveted butt joint, as generally obtaining in ocean steamships, is quite ten per cent. stronger than the best double-riveted joint, and is twenty per cent. stronger than the ordinary double-riveted lap

FIG. 62.-ORDINARY DOUBLE-RIVETED LAP JOINT.

joint. This means that at least fifteen per cent. in weight of shell could be saved without any sacrifice of strength.

To make this clear, let the accompanying drawings show an ordinary double-riveted lap joint, an ordinary triple-riveted double butt strap joint and the most approved triple-riveted double butt strap joint, respectively. The per

centage of strength of joint, as compared with the solid plate, is 59 for double-riveting, 72 for ordinary triple, and 83 per cent. for the improved compound triple, as per Fig. 64. With regard to this butt, it has been objected that eight and a half inches pitch is too great for solid caulking, but in reply we may state that the correct proportions of these very dimensions of joint is

FIG. 63.-ORDINARY TRIPLE-RIVETED DOUBLE BUTT STRAP JOINT.

placed beyond the region of doubt, as many such are now afloat on the ocean, after having been tested to three hundred and twenty pounds hydraulic pressure per square inch, without showing any signs of leakage or distress, and now carry one hundred and sixty pounds constant working pressure above the atmosphere without leakage always excepting the inevitable bottom leakage, due to expansion and contraction.

For any given plate and any given size of rivets, there is always only one proper pitch, which cannot be departed from without a sacrifice of strength. Fig. 64 and table marked A is drawn to show the possible gain by a correct system and proportion of riveting. It commences with an ordinary triple-riveted joint, in accordance with Lloyd's Register rules, and shows that the strength of plate at butt is fortyfive per cent. less than the section of solid plate; an improved triple butt with three complete rows of rivets, reduces the loss to thirty-five per cent.; with a compound triple and larger rivets, the loss is further reduced to twenty per cent.; and lastly, this loss amounts to only five per cent., the strength of riveted joint being now equal to ninety-five per cent. of the solid plate.

The sheer strakes and upper stringer plates of long, large vessels should always have double straps, and the bilge strakes may be treated in the same way with advantage; for the gain in strength is great, and the loss in speed by augmented surface friction is unappreciable. We assert this after having had outside straps fitted on the bilge and bottom of many steamers, of high and low speeds, for continuation of class in the Veritas and Liverpool Lloyd's Registry. It is said that these straps do not look nice

-a common expression, but one, we venture to think, which ought to be used with a greater

degree of caution by engineers, for to my mind what is nice looks nice.

Diagram No. 65 and table illustrate, on the left-hand side, the proportionate loss sustained by various thicknesses of plates, from threeeighths to one inch, with riveting in consonance with Lloyd's Register rules. On the other hand, it shows this loss largely reduced by adopting a proper system of riveting, whereby fifteen pounds

FIG. 64.-IMPROVED COMPOUND TRIPLE-RIVITED DOUBLE STRAP JOINT.

of plate are made to exceed twenty, twenty-five exceed thirty-five, and thirty-five pounds still more largely exceed forty in strength; the average gain being over 28 per cent., and that without any sacrifice of strength.

These percentages are calculated for countersink riveting, which, we need hardly say, is quite

a different thing to parallel holes; for, though the countersinking increases the size of rivet hole some twenty-five per cent., and thereby reduces the strength of plate for a given pitch, it adds nothing to the shearing strength of the rivets. It is, indeed, a premium that has to be paid for flush riveting.

The annexed table (B) shows the size of pitch, butt straps and description of riveting to obtain from 80 to 83 per cent. strength of joint, as compared with the solid plate for various thicknesses from three-eighths of an inch to one inch. This table shows that in riveted joints of structures composed of malleable iron or mild steel plates in which the joint is the means of the continuity of strength throughout the structure, that a reduction of about twenty-five per cent. might safely be made in an ordinary steel steamer, of, say, 2,500 tons gross register, if the arrangement of butts were good, the rivet holes in vital parts absolutely fair, and the rivet area and pitch of correct proportions.

It would further appear from this, that about 150 tons weight might be saved in steel plates of ships and boilers, worth, say, fifteen thousand dollars, and the dead weight ability of the vessel be augmented some five per cent., or with the same weight of metal the length and breadth of the vessel could be materially increased, whilst retaining the original factor of safety.