When the ship was in dock, on blocks perfectly straight, she came down in the midships, by the sights placed on the gun deck, two feet two inches and a half; and by those on the upper deck, two feet three inches and a quarter; and when undocked, with the trusses complete and in their places, she hogged, or broke her sheer, by the sights on the gun deck, one foot two inches; and by those on the upper deck one foot two inches and five eighths; and at the expiration of hours she had hogged, or further broke her sheer. two incites and Jive eighths, and then appear ed stationary and completely borne by the trusses.
We then proceeded to take away the trusses in the hold, and when they were wholly disengaged, she further hogged, or broke her sheer, six inches. We next proceeded to take away the trusses in the ports, and when they were wholly cleared, she dropped at the extremities, or further hogged, three incites and a half, and was in the same position when tried twenty four hours after.
We further beg to state, that the whole of the truss es alluded to as placed at right angles to the first in troduced, shalencd 118 the 8/rip floated from the blocks, and became short from half an inch to three inches and a half, and partook of no part of the pressure; which, in our opinion, clearly proves that the direction in which Sir Robert Seppings has applied his diagonal frame is correct, as also the great utility of the truss ing system; for although the ship, from her very de fective state, was much against so severe an experi ment, it has proved to us its good effects most satisfac torily; for many of the trusses in the ports forced the timbers three eighths of an inch within the ends of their covering planks, thereby lessening their effect from what it would have been if the ship had been of a sound texture; yet on a ship in this state, the trussing between the ports alone, after those in the hold were wholly disengaged, had the effect of sustaining the immense pressure of both ends of the ship in her worst position, and prevented her from breaking, which she otherwise would have done, from three to four inches, and which she actually and immediately did on their being disengaged." This statement of the Portsmouth officers, says Sir Robert Seppings, will, I trust, be considered conclu sive as to the benefits to be derived from the isle of trussing in the construction of ships though it was only applied from the keelson to t in the hold, and not to the ribs or frame of tin is the case when ships are regularly built on LlIlb oy 3.• tern, yet it sufficiently establishes the soundness of the principle.
When the Justitia first floated, continues Sir Ro bert, after being partially trussed, as described, the noise occasioned by the pressure on the trusses, is stated to have been truly terrific, until she was fairly settled on them. The disengaging them also caused a similar crash.
Very recently, Mr. Harvey has shown from theore tical principles, that Sir Robert Seppings's disposi tion of the diagonal frame is consistent with the soundest principles of mechanical truth. His inves tigation, originally published in the 42d number of the Journal of the Royal Institution, is here given.
Mr. Harvey introduces his subject with the well known mechanical principle, viz. that if through the point in which the sustaining forces meet, a line be drawn to represent the measure and direction of the straining force, and on it a parallelogram be construct ed, as a diagonal, having its sides parallel to the sus taining forces; then if the remaining diagonal of the parallelogram be drawn, and through the point where the sustaining forces meet, another line parallel to the same, all the parts of the framing on the Mlle side of this line, as the straining force, will be in a state of compression, and all those on the other side of the same line in a state of extension.
Mr. Harvey illustrates the application of this me chanical principle by two diagrams as follows: In Fig. 7. Plate CCCCXCV. let AB and AC represent two of the braces or ties of a system of a diagonal framing, and GD, DF, IIE, EF, corresponding- trusses. Let also GH, DE, and BC denote the longitudinal timbers of the same system, and F the fulcrum on which the whole is supported. Then if we apply the mechanical principle in the first place to the brace AC, and the longitudinal timber BC at the point C, where these timbers may be supposed to meet, let the vertical line CI be drawn to represent the measure and direction of the straining force operating at that point. On Cl as a diagonal, let the parallelogram Ca I b be constructed, having its sides in the direc tions AC and BC of the longitudinal axes of the tim bers selected for consideration. Draw the other dia gonal a b of the parallelogram, and through C, where the vertical force is supposed to operate, draw d e parallel to a b. Then, since the longitudinal timber BC is on the same side of d e as the straining force CI, it will, by the principle referred to, be in a state of compression; and the brace AC, being on the op posite side of the same line, will be in a state of exten sion.