Chap. XI.

MECHANICS.

955

The water from a reservoir enters through the pipe a, and tends to rush out through theannular orifice formed by the diaphragm and the edges of the circular orifice dd, with thevelocity due to the weight of a column of water of a constant and equal height: the dispersionof the annular orifice through which the water endeavours to escape increases the effect ofthe ordinary contraction of the fluid vein, and forms a cone whose summit entering a certainportion of the tube b, determines the distance which should separate the two tubes a,b.

On the diaphragm a small cone,/, of water will be formed; the water rushes into the pipeb , and remounts to a height equal to h, when the pipe is cylindrical. But if it be conical,it rises to greater and variable heights by reason of the qualities of the cone; arrived at itsmaximum of elevation it descends, introduces itself into the stationary cone of water, causesthe fluid vein to diverge, and obliges it to rush beyond in a parabolic curve through theinterval which separates the two tubes until the tube b is entirely full.

The directions of the jet d’eau return to their former position; then the contraction ofthe vein takes place, and the first effect recommences; this alternate motion takes place inperfectly equal intervals of time; but without the diaphragm the machine would produceno effect.

Continuous rectilinear motion with a uniform velocity, or varyingaccording to a given law , may be changed into a continuous circularmotion with a velocity of the same nature with that of the motion whichproduces it, constant or varying according to a given law, in the sameplane or in different planes.

A Cylinder turning on its axis, and a cord which envelopes its sur-face, gives the solution of this problem and that which is reciprocalto it.

An endless Chain furnished with teeth, which engage a toothedwheel attached to the cylinder, has been substituted for the cordemployed in the preceding figure.

Nut and Screw , as applied to the Press. — If on the samecylinder two screws be traced in a contrary direction, they willat the same time communicate two rectilinear motions in op-posite directions to the two nuts (female screw).

If the nut be fixed and the screw turn, the screw will havea rotary and rectilinear motion, which is employed for pene-trating hard bodies, for uniting them together, for raisingheavy weights, &c. M. Zureda contrived a very ingeniousmachine of this kind, where a cylinder turning round its axisby means of a handle ; on the surface of the cylinder twoopposite grooves were cut, like the threads of a screw, andwhich united or ran into each other at both ends of the cy-linder. A rod running along the top of a frame, with its otherend in the groove, was made, by turning the handle, to followthe spiral direction of the grooves, until it reached the end ofthe groove, where it communicated with the opposite groove;it then returned through it to its original position.

When the screw of the machine in the figure turns withoutchanging its place, the nut must not turn, but have a motiongiven to it in the direction of the screw, and in this case thecircular and rectilinear movements are divided. Fig. 1332.

The screw is one of the most useful machines in the arts; it is an auxiliary in almostevery machine, and readily changes form wherever it may be required ; a system of twoparallel screws is often used at the edges of a long plank to communicate a rectilinearmovement, and preserve its parallelism ; and the same principle, applied in an oppositedirection, forms the press. When motion is to be generated, the male and female screw arealways joined ; that is, whenever the screw is to be used as a simple mechanical power ;when joined with an axis in peritrochio,there is no occasion for a female, but inthat case the screw becomes a portion of acompound engine.

Circular Motion transformed to rectilinear ,as illustrated by Prony. gh is an axisdivided into three parts, a b, cd, ef ; the twoscrews ab,ef, are of the same gauge, theytraverse in two fixed supports k,n, in whichare two female screws; this axis moveshorizontally, and passes over at each turna space equal to one turn of the screw ;t d is another screw, whose distance b.-

Fig. 1330.

Fig. 1331.

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