The Toggle or Knee Joint.
Another most useful invention now comes before us, called the Toggle-joint, or Knee-joint, the latter name being given to it on account of its manifest resemblance to the action of the human knee.
This joint is shown in the illustration. It consists of two levers, jointed together at one end, and having the other ends jointed to the objects which are to be pressed asunder. It will be seen that if the centre of the Toggle be pushed or pulled in the direction of the arrow, so as to straighten the levers, the amount of pressure upon them is enormous. Such an apparatus as this combines simplicity and power in a wonderful manner, and is greatly used in machinery, especially in presses, where the force is required to be great, but not of long duration.
An ordinary two-foot rule, when bent, affords a good example of the Toggle-joint, and will exert a wonderful amount of force.
The illustration represents one of the common printing-presses that are worked by hand. When the workman draws the handle horizontally, he causes the two portions of the Toggle to approach a straight line. The upper half of the Toggle being jointed to the fixed beam above, and the other half to the movable plate or “platen” below, it is evident that the latter will be pressed downwards with enormous force. Indeed, so great is the power of this instrument, that a man of moderate strength can exert a pressure of many tons.
We now proceed from Art to Nature, and take first the human knee, being the joint from which this piece of mechanism has derived one of its names.
If the reader will look at the figure of the fencers, he will see that the arm and leg are both Toggle-joints. In the one who is standing on the defence they are bent, and in the other, who has just made a longe, the Toggles of the right arm and left leg are straightened. It is by the straightening of these joints, and not by the action of stabbing, that the rapidity and force of a thrust are achieved.
It is just the same in boxing. No one who has the least knowledge of sparring strikes a round-handed blow, for, putting aside the ease with which it is parried or avoided, it has scarcely any force in it. When a boxer hits “straight from the shoulder,” he not only straightens the Toggle-joint of his left arm, but that of his right knee also, so that the force of the blow comes quite as much from the leg as the arm.
It is by the right use of this joint that a small man, provided he be an expert boxer, will easily conquer an ignorant opponent who far surpasses him in size and weight. I have seen in a sparring-match a man not only knocked down, but fairly lifted off his feet, by a blow from a smaller opponent. The blow took effect under the chin, and, as the boxer hit exactly the right moment in straightening both limbs, a very great force was exerted with little apparent effort. I do not know which of the two combatants was the more astonished, the one to find himself on his back without exactly knowing how he got there, and the other to see his antagonist prostrate without exactly knowing how the thing was done.
The jointed apparatus by which the heads of carriages are raised or lowered is a good example of the Toggle, and exemplifies the force which a comparatively slight piece of machinery can exercise.
Another form of the Toggle-joint is the process called by sailors “bowsing” of rope. If a rope be fastened at both ends, and then pulled in the middle, the ends are drawn forcibly towards each other. This plan is mostly adopted in getting up sails. When a sail, say the mainsail of a cutter, has to be hoisted as far as it will go, the last few inches are always very obstinate. The word is then given to “bowse.” The rope, or haulyard, is no longer pulled at the end, but a turn is taken round the cleat, so that it does not give way. The rope is then forcibly pulled away from the mast, when up goes the gaff a little higher. In this way, by repeated bowsings, the gaff is coaxed, so to speak, up the mast, and forced into its place.
Some of the leaf-rolling caterpillars act in a similar manner, by alternately bowsing and shortening their lines. As, however, their mode of working will be described under another heading, we will say no more of them at present.
USEFUL ARTS.
CHAPTER II.
CRUSHING INSTRUMENTS.—THE NUT-CRACKERS, ROLLING-MILL, AND GRINDSTONE.—PRESSURE OF ATMOSPHERE.—SEED DIBBLES AND DRILLS.
Importance of Leverage in Crushing Power.—Nut-crackers a Lever of the Second Order.—The Chaff-cutting and Tobacconists’ Machines.—Jaws of various Animals.—The Wolf-fish or Sea-wolf.—The Rolling-mill and its Action.—Gunpowder-mills and Granulating Machine.—The “Jacob’s Ladder.”—The Mangle and its various Adaptations.—The Grindstone.—Primitive Grindstones of the Savage Races.—The Kafirs and the Inhabitants of Palestine.—Ceasing of the Millstone.—“Facing” of Millstones.—Tusk of the Elephant and its Structure.—Its Facings always preserved.—Power of Self-renewal.—Pressure of Atmosphere.—The Napier Coffee Machine.—The Cupping Instrument.—The Pneumatic Peg.—The Magdeburg Hemispheres.—Plane Surfaces of Glass or Metal.—Suckers of the Cuttle-fish.—Foot of the Water-beetle.—The Limpet.—The Star-fish and its Mode of Progression.—The Sucking-fish and the Fables connected with it.—Its real Structure.—Modification of the Dorsal Fin.—The Gobies and Lump-fish.—The Gecko and Tree-frog.—The Lampern and the Medicinal Leech.—Seed Dibbles and Drills.—Labourers versus Machinery.—Natural Dibble of the Grasshopper.—The Daddy Long-legs.—Drills and Dibbles of the Ichneumon-flies.—A wonderful Specimen from Bogotá.—The Pelecinus and its Mode of laying Eggs.