We owe this beautiful contrivance to the late Mr. Bramah. It is sometimes requisite that a wheel or axis should move with different velocities in different parts of one and the same revolution. This may be accomplished by an eccentric crown wheel acting upon and driving a long pinion. Thus, if the crown wheel in the margin rotates uniformly upon a centre of motion c,

which is not the centre of the wheel, and the teeth of this crown wheel play into the leaves of the long pinion p q, since the portions of the crown wheel pass in contact with the pinion with different velocities, as their distances from the centre of motion c vary, the pinion p q will turn with an unequable or varying velocity, depending upon the eccentricity of the centre of motion c.

3. Cams and Wipers. These are contrivances by means of which beams placed vertically, or inclined aslant upwards, may be made to advance over a small space in the direction of their length, and then recede in the opposite direction; and so on alternately. Eccentric circles, hearts, ellipses, portions of circles, and projecting epicycloids, serve communicate these kinds of motions. Thus, in the first of the figures below, the circular eccentric cam, being put into uniform rotation, the sliding or reciprocating part A B of the machinery, will ascend and descend with a gentle, smooth motion; being never at rest, unless at the very moment of changing its direction. In the quadrant cam, represented in the second figure, the reciprocating part A' B' will remain at rest on the periphery of the cam during the first quarter of the revolution, while, during the second, it will descend to

the axis of motion; during the third it will be at rest upon the axis; and during the fourth it will return to its original situation. The elliptical cam, in the third figure, turning upon

its centre, causes two alternate movements of A" B" for each revolution of the ellipse. In the fourth figure, a triple cam is applied to a tilt or trip hammer, turning upon a centre: there are three epicycloidal cams, or wipers, as in this case they are often called, causing three strokes of the hammer for one revolution of the wheel, to whose circumference these wipers are attached at equal distances. ds, banga

4. Parallel motions is a term given to the contrivances by which, especially in steam engines, circular motion, whether continued or alternate, is converted into alternate rectilinear motion, and vice versa. A moveable parallelogram is often, and very successfully, employed for this purpose; as will be described when we speak of the steam engine. From among the numerous other contrivances for this purpose, we shall select only one, which is very simple and elegant; and may be used in saw mills, and other

property, that if a circle rolls on the inside of another of twice its diameter, the epicycloid described is a right line. In practice, the piston rod, or other reciprocating part, may be attached to any point on the circumference of the wheel c.

m

6. Double Rack and Pinion.-This is a contrivance for an alternating motion with a gradual change. A B is a double rack, with circular ends, fixed to a beam that is capable of moving in the direction of its length. The rack is driven by a pinion P, which is susceptible of moving up and down in a groove m m', cut in the cross piece. When the pinion has moved the rack and beam until the end в is

B

reached, the projecting a meets the spring s, and the rack is pressed against the pinion. Then the pinion, working in the circular end of the rack, will be forced down the groove m m' until it works in the lower side of the rack, and moves the beam back in the opposite direction; and thus the motion is continued. The motion of the pinion in the groove will be diminished, if, instead of a double rack, there be used a single row of pins which are parallel to the axis of the pinion: this plan is sometimes adopted in the machines called mangles.

7. The Universal Lever.-This is a French invention, and is often, from the name of the inventor, called lever de la Garousse. It consists of a bar a b, moving upon a centre c, and having a moveable catch, or hook, hh' attached to each side, and acting upon the oblique teeth of a double rack; so that, as b and a alternately rise and fall in the reciprocal motion, the hooks h and h' successively lay hold of the teeth of the beam A B, and draw it up in the direction B A.

8. The Tachometer.-This is a very ingenious contrivance, which we owe to Mr. Donkin, for the purpose of measuring small variations in velocity. The contrivance is, in fact, hydrodynamical, but we mention it here, and the simplicity of its principle will render it easy of comprehension.

If a cup with any fluid, as mercury, be placed upon a spindle, so that the brim of the cup shall revolve horizontally round its centre, then the mercury in the cup will

assume a concave form, that is, the mercury will rise on the sides of the cup, and be depressed in the middle; and the more rapid the rotation of the cup, the more will the surface of the mercury be depressed in the middle and rise at the sides; the figures being those of hollow paraboloids. Now, if the mouth of this cup be closed, and a tube inserted into it, terminated in the cup by a ball-shaped end, and half filled with some coloured liquid, as coloured spirits of wine; then it is evident that the more the surface of the mercury is depressed the more the fluid in the tube will fall, and vice versa. Consequently, the velocity of rotation of the cup, and of the spindle to which it is attached, will be indicated by the height of the liquid in the tube; and, indeed, absolutely measured by it, when the apparatus has been subjected to the adequate preparatory experiments.

For a more minute description, see Trans. Soc. of Arts, vol. xxvi., or the article Tachometer in the PANTOLOGIA.

Two plates, exhibiting a great variety of contrivances for converting rotatory, reciprocating, and other motions, one into the other, and thus facilitating the construction of machinery, are given in my Treatise of Mechanics.

A large and valuable plate of the same kind, exhibiting 178 useful elementary mechanical combinations, has been lately published at Manchester, and is sold in London by Ackerman. I scarcely know a more interesting present than this would be to a young mechanic.

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CHAPTER XI.

HYDROSTATICS.

1. Hydrostatics comprises the doctrine of the pressure and the equilibrium of non-elastic fluids, as water, mercury, &e. and that of the weight and pressure of solids immersed in them.

2. DEF. A fluid is a body whose parts are very minute, yield to any force impressed upon it (however small), and by so yielding are easily moved among themselves.

Some attempt to give mechanical ideas of a fluid body by comparing it to a heap of sand: but the impossibility of giving fluidity by any kind of mechanical comminution.will appear by considering two of the circumstances necessary to constitute a fluid body: 1. That the parts, notwithstanding any compression, may be moved in relation to each other, with the smallest conceivable force, or will give no sensible resistance to motion within the mass in any direction. 2. That the parts shall gravitate to each other, whereby there is a constant tendency to arrange themselves about a common centre, and form a spherical body; which, as the parts do not resist motion, is easily executed in small bodies. Hence the appearance of drops always takes place when a fluid is in proper circumstances. It is obvious that a body of sand can by no means conform to these circumstances.

Different fluids have different degrees of fluidity, according to the facility with which the particles may be moved amongst each other. Water and mercury are classed among the most perfect fluids. Many fluids have a very sensible degree of tenacity, and are therefore called viscous or imperfect fluids.

3. DEF. Fluids may be divided into compressible and incompressible, or elastic and non-elastic fluids. A compressible or elastic fluid is one whose apparent magnitude is diminished as the pressure upon it is increased, and increased by a diminution of pressure. Such is air, and the different vapours. An incompressible or non-elastic fluid is one whose dimensions are not, at least as to sense, affected by any augmentation of pressure. Water, mercury, wine, &c. are generally ranged under this class.

It has been of late years proposed to limit the application of the term fluids to those which are elastic, and to apply the