1026
THEORY AND PRACTICE OF ENGINEERING.
Book II.
The beam which has one end placed in a wall and loaded at the other end will bedeflected from its horizontal position into an oblique direction, and Mr. Tredgold givesfor
Specific
Gravity.
Breadth
in
Inches.
Depth
in
Inches.
Deflexion
in
Inches.
Weightprodu-cing De-flexion inPounds.
Value ofconstantQuantity.
Oak
•922
4
2
2
1*176
112
•105
6
•537
4
2
2
1*34
112
•12
when a beam is fixed in a horizontal position, I L
The deflexion becomes greater as the distance A C is increased, and the equation given
— v V = D, and 0-6 D = B.
°' 6 / , , V S i
( b x cos. c x \\ \
L x--J
= D, and 0 6 D — B.
Tlie value of b for oak being *105, and for fir *12 ; the deflexion calculated as ^ inchper foot in length. A beam fixed in the before-men-tioned position has its fibre in the same state of tensionas when supported in the middle and loaded at eachend, its length being in one case double what it is inthe other : when resting on a fulcrum or support in themiddle, and acted upon by two weights at the end, it isalmost in the same condition as the force AB, exceptingthat it has a double strain in contrary directions. Tomake the necessary experiment, Mr. Peter Barlow em-ployed a block of hard wood, 18 inches long and 12 inchessquare, which was cut through at about 5 inches from Fig. 1566.
each end for the convenience of forming a mortice or
hole 2 inches square, and as much in depth ; the two parts of the block were then screw-bolted together, and an iron socket 2 inches square on the inside, made to fit the holes,was inserted in the mortice; the block was fixed by wedges into a wall, so that it wasquite firm and steady. The pieces of timber tested were reduced to a scantling of two inchessquare, and made to fit tight into the iron socket, upon the other end of which the weightwas suspended.
Of the Machines used to ascertain the Resistance of Materials to a crushing Force. — Theexciting force here acts directly in the reverse manner to that we have already described,and we have also to take into consideration the state of the material acted upon, whether itis too short to bend, as well as when its length is considerable with regard to its otherdimensions: in the first case, small pressure would crush it; in the other it might bend,and afterwards alternately break, by a force somewhat similar to that exerted by a trans-verse strain. Experiments were made upon the crushing force by Rondelet, which aredetailed in his Art de Batir; he states that a piece of oak whose base is an inch squarewould require 5000 or 6000 pounds weight to crush it, and a similar piece of fir wouldbear the additional pressure of another 1000 pounds.
Mr. Rennie found that a cube of English oak an inch square required only 3860 poundsto crush it, which result materially differs from the other, and leaves us still in considerabledoubt upon this subject. Screw-presses and long levers have been made use of for testingmaterials, and to ascertain the exact crushing force ; both methods might possibly be em-ployed with advantage, and the results calculated from the practical effects.
The Hydrostatic Press is a machine by which prodigious force can be obtained, and isfrequently used to cut through bars of iron, as well as to press paper, cloths, and othermaterials into a smaller bulk. A solid cross beam or piece of timber is fixed upon thetwo upright posts A, which are either let into the ground or framed into a sill at thebottom, and another, E, laid in an horizontal position, slides between the upright timbers;between this and the top the articles to be compressed are laid. The horizontal timber issupported on the under side by the piston D, which works in an air-tight cylinder F, at thebottom of which is a small pipe or tube b of much smaller bore ; the other end of this tubeenters at the bottom of a forcing pump L, and when the piston in the latter is pressed down,the water in the cylinder is forced under the piston which carries the horizontal timber andoccasions it to rise. The difference of the size or area of the surface of the small tube,compared with that of the rising piston, constitutes the power ; for if the sectional area ofthe tube be £ inch, and that of the rising piston 1 foot, as 576 to 1, or as 144 square inchesto ± inch : and supposing a pressure of a ton weight to be exerted on the handle H of the