a method in which the wheel is sustained by two floating cylinders.

CHAPTER IV.

ON THE ELEMENTS OF MACHINERY, AND THE CONTRIVANCES USED IN THE COMPOSITION OF MACHINES.

As every machine consists of a number of elementary parts, it is necessary that the mechanic should be acquainted with their nature and mode of action before he combines them together so as to compose a machine for producing a particular effect. The most important parts of machines may be arranged as follows:

I.-Account of Contrivances in which the Acting Parts have no Permanent Connexion.

1. Cylindrical Wheels and Pinions.-When a rod or axle is put into a rotatory motion by any power or first mover, the motion of this axle is conveyed to a second axle, parallel to it, by means of cylindrical wheels. If nothing more than motion was to be conveyed to the second axle, it might be done by two wheels, one placed on each axle, and having their cylindrical surfaces in contact. The mere friction of these circumferences.

would give motion to the second wheel, and the axle upon which it is placed.1 As it is necessary, however, to transfer the force as well as the motion of the first mover, the two wheels are generally made to communicate by means of teeth cut out of their respective circumferences, which act upon one another in the way already described.

If the second axle is to move with a different velocity, the wheel placed upon it must have its diameter one half the diameter of the first wheel, if it is to move with twice the velocity; one third if it is to move with thrice the velocity, and so on; or twice the diameter if it is to move twice as slow, and so on. In these cases the smallest wheel is generally called the pinion.

2. Bevelled or Conical Wheels.-When the motion of the first axle is to be communicated to a second axle, inclined at any angle to it, bevelled wheels (or bevelled geer) are employed, as already described, and the number of teeth in the one must always be to those in the other inversely as the velocities with which they are to move.

3. Crown Wheels.-In very small works, the motion of one axle may be conveyed to another at right angles to it, by means of a cylindrical pinion acting upon teeth at right angles to the radii of the wheel. Such a wheel is called a crown wheel.

4. Rackwork.-When the motion of an axle is to be used for giving a rectilineal motion backwards and forwards to a rod or beam, the machinery is called rackwork. . An example of this is shewn in Vol. I, Plate VI, Fig. 2, 3, Plate XIII, Fig. 4, and in Plate V, Fig. 9 of this volume.

5. A Ratchet-Wheel, or detent wheel, is a wheel fixed upon an axis which allows the axis to turn round in one direction, but prevents it from turning in the opposite direction. Ratchetwheels are generally employed to prevent a weight raised by a machine from descending.

1 In a saw-mill erected by Mr. Taylor of Southampton, the wheels act upon each other by the contact of the end grain of the wood. This method continued in use for twenty years. The wheels were of course made to bear against each other by some mechanical means. In small works a film of caoutchoc round the circumference of the wheels would enable the one to drive the other with great effect; or the same result might be produced in an inferior degree by rims of buff leather, similar to what Mr. Nicholson saw in the drawing of a spinning-wheel.

VOL. II.

N

In Plate XI, Fig. 1, Vol. I, a ratchet-wheel is shewn at N. (See also Plate II, Fig. 5). Its teeth have the shape of the point of a crescent lying in the same direction. When the wheel turns in one direction, a click such as O rolls over the convex surfaces of the teeth; but when the wheel attempts to move in the opposite direction, the click 0 presses itself, sometimes by the help of a spring, into the angular space between the two teeth, and prevents the wheel from moving.

6. Wheels with Belts and Ropes.-When the motion of one axle is to be conveyed to another at a distance, a wheel or pulley on the one is connected with a wheel or pulley on the other, by means of a belt or rope passing over each, as shewn at E F in Plate I, Fig. 12, Vol. I. When ropes are used, they lie in grooves cut in the circumference of each wheel; but when a belt is employed, it acts simply upon their cylindrical circumferences. When the belts become loose by being stretched, they lose their power of turning the axles, and in this case they must either be shortened, which is sometimes done by means of buckles, or the friction must be increased by chalk or other means. Sometimes this evil is remedied by having grooves on a pulley of different diameters in one or both of the wheels, and when the belt or rope becomes slack, it is shifted to the next largest circumference. If this, however, is not done on both wheels, the velocity of the machinery will be changed. Belts are often apt to quit the wheels upon which they work, which generally arises from the circumference not being accurately cylindrical. If the circumferences are conical, the belt has always a tendency towards the base of the cone, and hence it is usual to give the circumferences of the pulleys or wheels a barrel shape, or to make their diameter greatest in the middle, by which the belt always keeps its place. When the belt or rope crosses between the wheels, the second will move in a contrary direction to the first; but when they do not cross, the wheels will both move in the same direction.

7. Rag-wheels with Chains.-When the machinery is exposed to great strains, or when its movements require to be very regular, chains of various kinds are substituted in place of belts and ropes. These chains generally lay hold of pins or hooks, or enter into notches on the circumference of the wheel, and hence such wheels have been called Rag-wheels. Three different

kinds of rag-wheels are shewn in Plate VI, Figs. 3, 4, 5. In Figs. 3 and 4 the chains take hold of pins p, p, &c. projecting from the wheel, and in Fig. 5 there are sharp angular points on the chain AB C, which enter into notches t t upon the wheel's circumference. Another example of rag-wheels is shewn in Plate II, Fig. 11, at A and B.

8. Axles, Gudgeons, and Pivots.-Axles are generally fixed permanently to the wheels which they carry; but Mr. Brunell, in the block machinery at Portsmouth, connected the wheels with their axles solely by friction. A hollow conical shoulder in the centre of the wheel was fitted upon a conical part of the axle. By driving the interior upon the exterior cone, with a few blows from a hammer, the friction was sufficiently powerful to prevent the wheel from slipping round, while the machine was performing its ordinary work; but when the force applied to the wheel was increased by any unforeseen cause, this force, in place of crushing the machinery, overcame the friction between the two cones, and the wheel revolved upon its axle without turning it, and consequently without injuring the machinery. The Gudgeons are the metallic and cylindrical extremities of axles upon which they rest and turn. They should never be made much larger than the nature of the machinery requires. When the extremities of axles are conical, they are called Pivots. The hollows in which the gudgeons rest are called Bushes. A vertical gudgeon with its bush is shewn in Fig. 6 of Plate VI, and a horizontal one in Fig. 7. They should be made of hard substances of uniform texture, and plentifully supplied with oil or grease. Sometimes they are made of siliceous stones in mills. In wind-mills the great collet of the axle is sometimes supported in blocks of marble slightly hollowed. M. Borgnis has found that green oak that has been soaked in boiling oil is preferable to copper. When the wheel is subject to horizontal or vertical displacements, it is necessary that the gudgeon should have a cover m (Fig. 8), kept down by screws dd, the tail b diverging, in order to keep it firmer in the wood.

9. Apparatus for Locking and Unlocking Machinery.—In many machines it is necessary sometimes to connect one axle with another, and at other times to separate them. This is called

throwing in and out of geer, and may be effected in various ways. The most elegant method of doing this is to place upon one of the axes a conical shoulder, which can slide freely end ways, but is prevented from revolving by fillets or projections from the axle. This conical shoulder is forced by the power of a lever. into a hollow conical shoulder, and is rivetted with it so firmly by friction that they act as if they were firmly joined together. By the action of the lever, they are again separated at pleasure.

Another method of throwing either of two wheels into geer is shewn in Plate VI, Fig. 9, where A and B are the two axles, and P, O the wheels fixed upon them. When it is required to throw the wheel O into geer, the clutch q is pushed by the arm fe into hollows in the face of the wheel, and the same is done in the opposite direction by the clutch q when P is to be thrown into geer. In the position shewn in the figure, the wheels are both out of geer. This contrivance is part of a self-regulating sluice, which is elevated or depressed to admit more or less water according as the pinion O or P is thrown into action. See Edinburgh Encyclopædia, Art. Hydrodyna mics, vol. xi, p. 561.

10. Endless Screws.-The object of an endless screw is to communicate the motion of an axle to a wheel lying in a plane passing through the axle. It is shewn at H in Plate I, Fig. 9, where it works in the teeth of the wheels C, H. In this construction of the endless screw only a few teeth of the screw are engaged at the same time; but when great strength and solidity are required, the wheel in which the endless screw works has a groove ab (Fig. 10) in its circumference, like a female screw. By this means many teeth of the screw and the wheel are engaged, and the construction is remarkably solid. This contrivance is used in the dividing engines of Ramsden and Troughton, and is well represented, both as applied to rectilineal and circular motions, in the Edinburgh Encyclopædia, Art. Graduation, Plate 281, Figs. 7 and 8.

11. Lever of Lagaroust.-This contrivance, which produces a rectilineal from a circular motion, is represented in Plate VI, Fig. 11. The lever AB has a motion round an axis C, in a fixed beam M N, the use of which is to elevate or depress the toothed rack FG. This is done by the two secondary levers DE,