980

THEORY A XU PRACTICE OF ENGINEERING.

Cook II.

We may deduce the means of tracing a curveam np, cut in a groove, and fixed to a lever b b,which may turn freely round an axle which traversesits extremity b : if we suppose that the wheel m turnsuniformly round its centre, and the pin p , fixed toa point at the surface of the wheel o , enters the groovewhich forms the curve a mnp: this curve may besuch that the lever bb makes oscillations which fulfilone of the following conditions.

lurst, that the arcs described by any point of therule bb should be described with a uniform velocity.

Second, that the velocity varies according to a given law.Third, that the arc should not be described, butits chord, which should be gone over with a uni-form velocity, or variable according to a given law.

Tig 1152.

The curve drawn in this figure may be traced in such a manner as to satisfy the firstof these conditions.

We may also fix the curve ede/tothe surface of the wheel m, so that the circular uni-form motion of the wheel m communicates to the lever ab a motion of oscillation whichmay satisfy one of the three conditions just explained in the preceding motion, by meansof a pin, p, fixed to a lever which engages the groove. The curve drawn in this figureanswers to the first of these three conditions. The means indicated in this article and thatwhich precedes arc capable of different applications useful in the arts. 'Hie circular alter-nate motion of the lever belongs to this class ; but if it is the chord of the arc drawnon that on which we may fix a rule pierced by a groove, the intersection of this grooveby another longitudinal one made in the lever offers a vacant point in which we mayintroduce a point, which may have an alternate rectilinear motion : the thing will happen ifit is the motion of a weight suspended at the extremity of a rule by means of a cordpassing through a pulley.

A cylinder, a, furnished with cams, raises the hammer V, which turns on its axlec;this is a well-known motion, and requires no further explanation.

a is the extremity of an axle ofa great wheel or fly, furnished witha ratchet-wheel b: cc is a wheelwhich fits the axle of the fly; itcarries the catch d , which arreststhe ratchet-wheel by means of aspring ; the circular alternate mo-tion of the wheel c will transmitto the fly, whose axle is at a, a cir-

cular continuous motion in the same Fig 1421.direction, but the wheel c will only act during half its oscilla-tions.

There is another application of the preceding motion ; pq isa balance beam with a circular alternate motion, which it com-municates to the wheel o by means of a cord abed. It isstretched by the weight e, or by a spring : the wheel o fits theaxle h of the fly; the extremity of the axle of the fly isfurnished with a ratchet-wheel, to which a catch g hooks, fixedin the wheel o .* the circular alternate motion of the wheel ocauses the fly to turn in the same direction.

6 is a balance capable of an alternate circular motion roundthe axle c: d is a rod which turns freely on its axle at c; atits extremity is a toothed wheel, e , which engages another,^fixed to tlie axle of the fly, n. In the opposite face of the twotoothed wheels e and f is a rod, g , which forces the wheel e topreserve this same distance from the centre of the axle ofthe fly. The alternate circular motion of the balance bcompels the wheel e to ascend and descend, but it canonly ti;ke place when the wheel f turns on its centre; ac-cording to the form of the machine the motion may be con-tinuous or alternate, but the inertia of the fly necessarilyrenders it continuous and nearly uniform; the reciprocal takesplace when the motion has begun : in steam engines thismechanism is employed, known by the name of the fly: it mayeasily be conceived, that notwithstanding the two wheels e andf arc of the same diameter, the fly n should make two revolu-

Fig. 1425.

Fig. 1427,