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means, when the screw is turned round, the saddle-piece will slide uniformly along the triangular bar A.

“ K is a small triangular bar of well-tempered steel, which slides in a groove of the same form on the saddle-piece r. The point of this bar or cutter is formed to the shape of the thread intended to be cut on the endless-screw. When the cutter is set to take proper hold of the intended screw, it may be fixed by tightening the screws (e), which press the two pieces of brass G upon it.

Having measured the circumference of the dividing-wheel, I found it would require a screw about one thread in a hundred coarser than the guide screw h. The wheels on the guidescrew arbor , and that on the steel E, on which the screw was to be cut, were proportioned to each other to produce that effect, by giving the wheel L 198 teeth, and the wheel a 200. These wheels communicated with each other by means of the intermediate wheel R, which also served to give the threads on the two screws the same direction.

“ The saddle-piece p is confined on the bar a by means of the pieces (g), and may be made to slide with a proper degree of tightness by the screws (n)."

For other excellent observations and directions relative to the dividing of instruments, see Mr. Smeaton's paper containing an account of Mr. Hindley's method, Phil. 'Trans. vol. Ixxvi. or New Abridgment, vol. xvi. p. 30—56, and the papers of Mr. Troughton, Mr. H. Cavendish, and Professor Lax, in Phil. Trans. for 1809, abridged in the Retrospect of Phil. and Mech. Discoveries, No. 23.

REVERSING OF MOTIONS, contrivances for. We do not here mean to speak of alternating or reciprocating motions after intervals of short continuance, those being already treated of in the introductory part of this volume, also under the title PARALLEL motions, besides that they occur incidentally in the separate descriptions of several machines. We shall now mention some methods of reversing motions after much longer intervals; as in the case of drawing up buckets from wells or mines, where no change of direction may be required for several minutes; or in different kinds of mill-work, where the direction may not be changed for some hours.

Contrivances to effect such reversion of motion are very numerous ; but almost all of them may be reduced to two general methods: for the required change is generally produced either by making two equal pinions on one and the same axis take alternately into the teeth of those parts of a larger wheel which are nearly diametrically opposite; or, by means of an

additional wheel which may, as the practical mechanics term it, be thrown in and out of gear alternately.

In many engines for drawing buckets out of mines that are moved by horses, the motion is frequently reversed by turning round the animal, and causing him to retrace his steps and draw the contrary way: but this is found very injurious to the horse, a circumstance which has frequently led to the adoption of other methods. In Emerson's Mechanics a simple contrivance is described; consisting merely of a horizontal facewheel upon the same vertical shaft as the horsepole is attached to, and two equal pinions upon the same axle as carries the drum or barrel on which the rope winds. The axle which carries the drum and pinions is fixed horizontally, a little above a diameter of the face-wheel ; and first one and then the other of the pinions is made to be driven by that wheel; thus mani. festly reversing the motion as required. There are two methods of attaching these pinions to the axle, and making them to be acted upon by the face-wheel: in one of them, the pinions are fastened upon the axle at a distance from each other exceeding the diameter of the face-wheel only 3 or 4 inches; then, the axle being moved horizontally through this small distance brings first one and then the other pinion into contact with the wheel at opposite extremities of a diameter, and thus changes the direction of the motion ; but this method is attended with the disadvantage of having often to move a heavy weight with the horizontal axle, besides that there is much danger of breaking the teeth of the pinions and wheel when they first come to embrace each other. `In the second method, the lanterns or pinions both turn constantly with the face-wheel, but they play freely upon their common axle, except they are stopped by a pin which fixes them; the application of such pin to first the one and then the other of the lanterns produces the alternating motion as proposed.

M. Prony has two contrivances for reversing the motion in horse-whims, without changing that of the animal : in both of which, however, the general principle is the same as that adopted by Mr. Emerson. In the first a horizontal wheel, toothed at its face, lay just above two vertical pinions, fixed on the opposite extremities of an axis of the length of its diameter. This wheel was so contrived as to incline a little from its hori. zontal position to either side at pleasure; so that on the one inclination its teeth locked with those of one pinion, and receded from the other; and on the other position, its operation on the pinions was reversed: by which the axis of the pinions turned round first in one direction, and afterwards in the contrary.

M. Prony, finding this method subject to some inconveniences, contrived the following, which he esteems much superior to it. An horizontal wheel, toothed at its face, and attached to a perpendicular arbor (which gives it motion), turns two pinions, moveable on the same axis, which it meets at the opposite sides of its circumference: these pinions are not attached to the axis, but turn round freely upon it: the intermediate part of the axis is square, and has adjoining to each pinion boxes which slide back and forwards on it, each of which supports a faced wheel, with strong serrated teeth ; the serration being in a different direction on the opposite wheels : the boxes are connected by two iron bars, so as to change their places by one movement; to the pinions there are also serrated faced wheels attached, so as to lock on those opposite to them on the sliding boxes. From this construction it follows, that when the boxes are slidden to one extremity of the axis, the pinion at that side will be connected with the axle, and communicate its motion to it in one direction; and when the boxes are moved to the other extremity, then the first pinion will be disengaged, and the second be locked to the axle, and cause it to turn round in a direction the reverse of that in which it moved before. There is a lever on another axle, whose office is to move the before-mentioned boxes backwards and forwards: an arm projects from the axis, which moves between two pieces, proceeding from the frame connected with the boxes : the lever rises upwards, and has a weight at its top, by which it presses strongly in either direction, when it passes the perpendicular position; forming thus the contrivance vulgarly called a tumbling-bob, which is used in various engines for a simular purpose. Upon the same axle on which the pinions move is fastened a drum-wheel, round which passes the chain or cord to which the buckets are attached; another chain or cord is placed below the buckets, from the bottom of one to that of the other, to form an equilibrium between the whole of the appendage of one bucket and that of the other in all positions. A bar is so placed, that, on one of the buckets rising to a certain height, it catches the bar, forces it upwards, and thereby throws over the tumbling-bob connected with its other extremity : this reverses the movement of the buckets; and, on the other bucket rising, it operates in the same way on another lever, which throws the bob to the other side, and causes the first bucket to rise again.

M. Prony has annexed a contrivance to this engine by which the horse that puts it in motion is disengaged when any accident happens, which would tend to stop the movement of the wheels : for this purpose the traces pass under two pulleys in

VOL. II.

AA

the ends of the yoke; and their extremities, which have loops wrought in them, are alternately attached to two pins in a roller, round which a cord is wound two or three turns, and passes from thence through rings in the lever, which causes the arbor to revolve, and over a pulley on the arbor to a weight which hangs beside it. When the draught exceeds this weight, it is evident the roller will be drawn round by the traces, and that they will slip off the pins, and be disengaged during the first revolution.

The method of reversing motion by causing pinions to be operated upon by the opposite parts of a face-wheel has been long known and practised by millwrights; and they have various contriyances for performing the alternation, as by levers, screws, tumbling-bobs, &c. One of these will be illustrated by a figure, when we come to the article TID E-mill. And for several methods of reversing motions, see pl. XXXVIII. and XXXIX.

As to the second general method, it has perhaps an appearance of greater simplicity; though, when reduced to practice, it is commonly found more expensive than the former. Suppose that while the horizontal wheel A (fig. 2. pl. XXVIII.) continues to turn always one way, it is required to have the horizontal wheel B turn, sometimes in one direction, and sonietimes in another : by means of an additional wheel c, equal in diameter and number of teeth (supposing the velocities in both directions to be equal), this may be accomplished, thus : Let the two wheels B and c have the lower pivots of their axles resting in boxes or cases that may be moved up and down by means of screws; and, while the wheels a and B are nearly of equal thickness, let the wheel c be somewhat more than double the thickness of either: when the motion of the wheel B is to be in a contrary direction to that of A, let the wheel c be lowered so much that its teeth play neither into those of a nor B, while the teeth of A take into those of B and drive it round: when, on the contrary, B is to be moved in the same direction as A, let the wheel B be lowered till its teeth do not come into contact with those of A, and let c be raised until the upper parts of its teeth take between those of the wheel a, while the lower parts of other teeth play into the teeth B; so shall the rotation of B have the direction required. If the motion of the wheel A were sometimes in one direction and sometimes in another, the motion of B might all along be preserved in one direction, by the occasional application of c as an intermediate wheel.

REGULATOR of descending motions. See Hardie's CRANE, page 173, and Harriot' s Engine, page 246.

ROTATORY MOTION, when produced by a reciprocating motion, requires some contrivance to render it uniform, or nearly so, The usual inethod of equalizing is by attaching a fly-wheel to some part of the machinery: but Mr. Arthur Woolf has invented an apparatus to be substituted for the fly in steam engines, which possesses the advantage of equalizing the motion, with the property of being stopped and set to work at any part of the stroke.

Plate XXIII. fig. 4. A represents part of the engine-beam; B the connecting-rod; c the crank arm; va cog-wheel, working into another cog-wheel E, of half the size; F a crank-arm on the shaft of the small wheel ; g a cylinder closed at bottom, in which a solid or unperforated piston moves, leaving a vacuum beneath. This acts simply instead of weight on the crank F, by the constant pressure of the atmosphere ; and the diameter of the piston must be such as nearly to equal one-third of the power

of the engine. In fig. 5. the outer circle is the line described by the crank; the circumference of the inner circle is equal to twice the diameter of the outer, and the square has the same circumference: this last exhibits the inequality still remaining, which by this method is reduced to about one-fifth ; but by the assistance of a small fly on the second motion, the effect will become nearly the same as that of a rotative engine with the advantages bere mentioned.

The same motion may be applied to a pump, but in this case the two cranks must be horizontal at the same time. (Nich. Jour. No. 29. N.S.)

SAWMILLS, constructed for the purpose of sawing either timber or stone, are moved by animals, by water, by wind, or by steam. They may be distinguished into two kinds: those in which the motion of the saws is reciprocating, and those in which the saws have a rotatory motion. In either case the researches of theorists have not yet turned to any account : instead, therefore, of giving any uncertain theory here, we shall proceed to the descriptive part, and refer those who wish to see some curious investigations on this subject to a Memoir on the Action of Saws, by Euler, in Mem. Acad. Roy. Berlin, 1756.

Reciprocating sawmills for cutting timber and moved by water, do not exhibit much variety in their construction. The sawmill represented in pl. XXVIII. is taken from Gray's Experienced Millwright; but it only differs in a few trifling particulars from some which are described in Belidor's Architecture Hydraulique, and in Gallon's Collection of Machines

approved by the French Academy. 1. The plate just referred to shows the clevation of the mill,

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