LEVU:, or LEAVER (from the French levier, formed of the verb lever, derived from the Latin, levare," to raise") in mechanics, an inflexible straight bar, supported, in a single point, on a fulcrum, or prop, and used for the raising of weights.
The lever is the first of those called mechanical powers, or simple machines, as being, of all others, the most simple ; and is chiefly applied to the raising of weights to small heights.
In a lever three things are to be considered : the weight to be raised, or upheld ; the power by which it is to be raised, or sustained ; and the fulcrum, or prop, by which the lever is supported, or rather on which it moves round, the fulcrum remaining fixed.
Levers are of three kinds : sometimes the fulcrum, or centre of motion, is placed between the weight and the power. This is called a lever of the first kind, or vectis heterodromus ; to which may be reduced scissors, pincers, snuffers, &c. : sometimes the weight is between the fulcrum and the power, which is called a lever of the ,second kind ; such are the oars and rudder of a boat, the masts of ships, cutting knives fixed at one end, and doors whose hinges are as the fixed point : and sometimes the power acts between the weight and the fulcrum, which is the lever of the third kind ; such is a ladder lifted by the middle to rear it up against a wall : these two are called vectes homodromi.
In this last, the power must exceed the weight in propor tion as its distance from the centre of motion is less than the distance of the centre from the weight. And as the first two kinds of lever serve for producing a slow motion by a swift one, so the last serves for producing a swift motion of the weight by a slow motion of the power. It is by this kind of lever that the muscular motions of animals are performed, the muscles being inserted much nearer to the centre of motion than the point where the centre of gravity of the weight to be raised is applied ; so that the power of the muscle is many times greater than the weight which it is able to sustain.
Though this may appear at first a disadvantage to animals, because it makes their strength less ; it is, however, the effect of excellent contrivance ; fur it' the power were, in this case, applied at a greater distance than the weight, the figure of animals would be not only awkward and ugly, hut altogether unfit for motion ; as Borelli has shown in his treatise De Motu Animalium.
The knowledge of the properties of the lever is of the utmost trse in ascertaining the laws of the resistance of tim ber ; we shall therefore begin with the first principles of motion, from which the properties of the lever are obtained ; and also the principles of the centre of gravity of one, or of a system of bodies.
1. Force is the power exerted on a body to move it.
2. Direction of motion or tendency is the effort which one body makes to move another towards a given point.
3. Line of direction is the straight line in which a body moves, or has a tendency to move, without having any regard to the point to which it tends.
4. Angle of direction is the angle contained between two lines of direction.
5. When two or more bodies act against each other with out any of them being overcome by the rest, this state of quiescence is called equilibrium.
6. Opposite directions, or opposite tendencies, are when each of two bodies move, or have a tendency to move, to a different point in the same line of direction.
7. Opposite forces are those that act upon each other in the same line of direction, but have a tendency to contrary points in the line, by which tendency an equilibrium is produced, or otherwise a change of motion.
S. Contrary directions are when two bodies move, or have a tendency to move, in lines parallel to two opposite planes.
Axiom 1.—Every body endeavours to preserve its present state, whether of rest or of moving uniformly in a right line, till it is compelled to change that state by some external force.