Chap. XIII.
BOATS, ETC. FOR TRANSPORTING WEIGHTS.
1025
stability is required to be increased, the nearer the stones are deposited to the middle of theship, in the full parts of the body, the more effectually will this be accomplished.
Fig. 1565.
BOATS POR THE BREAKWATER.
Machines, §c. for proving the Strength of Materials , differ in their arrangement accordingto the strains that are to be tested. We shall consider, first, what materials may be drawnasunder by a force acting at the end ; secondly, how they may be compressed by a force actingin the same direction ; thirdly, how they may be strained laterally, one part being supported,and the strain immediately applied at the point of support ; fourthly, how they may besimilarly affected transversely ; fifthly, how they may be twisted ; sixthly, how they may bestrained by any two or more of the above forces, and also how they may be strained by aninternal pressure, as is the case with hydraulic cylinders, water-pipes, &c.
First, with regard to the resistance to extension in length, arising from the direct cohe-sion of the fibres or particles of matter ; this kind of strain in timber requires some care tobe observed in ascertaining it, as the fibres are liable to be injured or destroyed whenroughly experimented upon : one of the best machines for the purposes here required isdescribed by Mr. Peter Barlow , in his Treatise on the Strength of Materials, and who madeexperiments with it upon a piece of wood 12 inches in length, the square ends of whichwere 3£ inches, and the side of the square 1^ inches; the intermediate part, 5 inches inlength, was by means of a lathe reduced to a third or a quarter of an inch in diameter, theother cylindrical portions being left | inch in diameter; these pieces, so prepared, wereplaced at the top of the frame, and passed partly through a circular hole in an iron frame,laid horizontally upon two upright supports; the sides of this frame were then screwedtightly together to prevent the piece of prepared wood being pulled through ; a couple of ironboxes were placed upon the lower square, and screwed tightly up ; near the top they hadtwo semicircular holes, correctly fitted to the larger part of the cylinder ; these were shut bypassing bolts through them, which perfectly secured the two shears ; the head of the pieceof timber was placed above the collar, the upper large cylinder being retained in the hollowiron chamber at top prepared to receive it; and the lower square end, inclosed in the twoiron boxes, the hook of the scale was passed through the circular hole, and the scale, beingwedged up from the ground, was loaded.
Among many experiments, the following results have been obtained, the smaller cylindersbeing reduced to what they would have been on square inch bars, it being assumed thatthe strength is proportional to the section of fracture r—
Box -
-
- 20,000 pounds per square inch.
Ash -
-
- 17,000
Teak
-
- 15,000
Fir -
_
- 12,000
Beach
-
- 15,500
Oak -
- 10,000
Pear
.
9,000
Mahogany
-
8,000
Of the Stiffness of Beams supported at one end: as it is difficult to fix a piece of timber inthis manner, so that it will not be liable to extension beyond the point of support, there is agreat difference in the results of the several experiments that have been made on the subject.
3 U