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THEORY AND PRACTICE OF ENGINEERING.

Book 11

the axle B is cut in a screw shape : E is an iron bar, in which the cutter INI slidesfreely ; this bar has the form of a square or triangular prism, it is sustained by twopuppets F and G : abed is a large and deep mortice practised on the plane whichsustains the puppets F, G, H, I: efbd is a rule whose thickness is equal to the depthof the mortice abed; it enters into this mortice, and slides freely throughout its length ;from the surface of the rule ef, bd two puppets K and L rise; the first is terminatedby a cylinder pierced with a circular orifice, in which the axle B B enters, and the second bythe nut of the screw gh : at the extremity ef of the rule efbd is fixed the rule esi; the rule i'kturns on a hinge round the point i, it has a longitudinal slit Im , in which a small cylinderenters, which carries the tool M : p and g are two small spheres which are at the extremityof the two cylinders which rise, the first from the rule efbd-, and the second from the ruleik: these two cylinders turn freely round their axes; the part no of the rod np iscylindrical and traverses the circular orifice of the little sphere p, it can turn freely butcannot slide ; the other part, or, of the same rod, nr, is cut in a screw shape, and enters thenut cut in the little sphere#; if the cylinder A A turns, its circular motion will betransmitted to the axle B B, the rule efbd will advance, and draw with it a rule ik; we mayvary at pleasure the inclination of the latter by means of the rod n r, consequently it will fulfilthe functions of an inclined plane, and will conduct the tool m with the required velocity.

Alternate rectilinear motion with a uniform velocity, or variable according to a given law,may be changed to circular alternate with a velocity of the same nature as that of the motionwhich produces it, constant or variable according to a given law, in the same plane or in differentplanes.

Fig. 1-471. a b is a lever turning on its axle; the centreof the semicircle r/e/* fixed to the same lever, atdand/arethe extremities of a chain dgjihf fixed to two bolts, in such amanner as to be shortened or lengthened at pleasure; thechain passes round two pulleys g and k , and the alternatecircular motion of the lever a b produces an alternate recti-linear motion at the point h of the chain. This motion hasbeen applied to a machine for cutting off the heads of pilesunder water, and is one of the most simple and convenient forthis purpose.

Fig. 1472. The zigzag is a very well known motion re-quently employed in toys: Du Vivier employed it in amachine for drawing boats out of the water: the nippers forraising heavy bodies from depths is a familiar illustration of it,as is also the common reel or spindle.

Fig. 1473. is a double suction pump invented by Noble,and first used on board the "Windsor Castle , 74 guns, in 1790.

It consists of a lozenge formed by four rods a,b,c,d of iron,united by a hinge at their extremities, in such a manner thatthe angles may become acute or obtuse without increasing ordiminishing the perimeter of the figure. The plan of thelozenge is here represented vertically, with one of its diagonalsin a horizontal position, and sustained by two pillars ofequal heights, in such a manner that the same bolt whichunites the contiguous sides attaches them to the correspondingpillar; in this position it is clear that the other diameter ofthe lozenge will be vertical; it should be in the same axis asthe pump barrel: at the upper extremity of this diagonal tworods of the lower piston of a pump arc attached, and at itsother extremity that of the upper piston, and the same boltwhich unites the contiguous bars to this diagonal unites themalso to the piston rods; since the angles of the lozenge must vary, and with them thelength of the diagonals: the two pillars e,e cannot be fixed; they are consequentlyplaced between two cheeks c,r, to which they are united by a bolt, and thus have a motionof rotation in the plane of the lozenge.

Sometimes it is required to communicate an alternate circular motion, like that of apendulum, by means of an alternate motion in a straight line: this is done in a remarkablemanner in the steam-engine, where the moving force is the pressure of steam, whichimpels a piston from one end to the other alternately in a cylinder; the power of thispiston is communicated to the working beam by a rod connected with one end of thepiston. The end of the beam, which has motion communicated to it when united with therod, draws it alternately to one side and tire other, moving in the arc of a circle, the centreof which is that of the beam : this being the case, it is necessary that the rod and the endof the beam should be so connected that, while the one ascends in a straight line, the othershall descend in like maimer.

Fig. 1471.

Fig. 1472.

Fig. 1473.