manufactures.

121

Manufac- the shaft D to be turned round in the direction of thetures. arrows, it is plain that the wheel B will revolve roundthe wheel Ain the same direction, and thereby elevatethe rod C C, till the wheel B is found at the top of thewheel A; when by continuing the motion of the largewheel, the wheel B must necessarily descend to its firstposition, and so on continually. This, however, as wehave stated it, is converting a rotary into a verticalmotion ; which was done, because in this form its actionappears to be more easy to comprehend. We have onlynow, however, to consider the rod C C to be first putinto motion, when it necessarily follows that it must byits action give rotation to the wheel A, and to the largewheel to which it is fixed.

It was by this means Mr. Watt converted the reciprocating motion of his first steam-engines into a ro-tary motion, not however entirely as a matter of choice ;he would have preferred the crank, but was preventedusing it by a patent which had been taken out by anotherEngineer. There are cases, however, in which it maybe still used with advantage, although the crank hasnow entirely superseded the use of it in steam-engines.

Rotary motion is transmitted and converted intorotary motion in the same or other direction in a greatmany ways, viz. by bands, straps, wheel and pinion,bevel gear, and with various changes of speed ; but theseare generally so simple and so common as to be familiarto every one, we shall therefore only select one or twocases, and those only where change of direction orchange of speed make them rather peculiar.

(76.) By bands or straps. One obvious, and perhapsthe earliest method of transmitting rotary motion, withor without a change of speed, is by bands or straps, ofwhich we have numerous common examples. Fig. 6shows a motion of this kind, in which by having a seriesof pulleys, or of grooves, in one common pulley, corre-sponding with others in another such wheel, so as to pre-serve always the same length of band, a great varietyof speed may be obtained by merely shifting the bandfrom one pair of opposite grooves to another opposite

P The bevel gear furnishes a method of changing thedirection of motion from one line to another, formingany angle with it. See what has already been said onthis head, section (33.); see also fig. 2.

(77.) Alternate cones are sometimes used for changingthe velocity of motion, when this requires to be graduallyand uniformly accelerated. Here one of the cones, A,(fi<r. 7,) gives motion to the other by a belt, which iscarried along by a guide, by which means the motion isaccelerated or retarded at pleasure, or retained constantwhile the guide remains stationary. .

Another method of changing velocity is shown in fig.8. The two wheels A and B are fixed on the same axis,a little apart from each other; the other two wheels,C, D, are cast together, and run on an axis with themeans of slipping them a little backward and forward;when C D is thrown back on its axis, the teeth of thewheel C engages those of the wheel A, while those ofB are thrown beyond the wheel D, and a certain speedis the result of this action; but by sliding the wheel Cforward, the teeth of the wheel B engage those of D,and a different velocity results, as is evident.

(78.) The conversion of continuous rotary motion intoa reciprocating rectilinear motion, may obviously beeffected by merely inverting the methods already de-scribed for converting a reciprocating rectilinear motion

into a continuous rotary one, but they are not com- Machineiymonly employed in practice. The two following have,however, been occasionally used for either purpose,accordingly as circumstances required. We shall herespeak of them as a means of converting a rotary intoa rectilinear reciprocating motion.

Conceive a pinion A, fig. 9, to be made to give Fig. 9 .motion to the wheel B B', and this again to give mo-tion to the equal wheel C C'; to the axles of these, beyondthe beam L L, are fixed the cranks D D', and to pinsat their extremities are the suspended rods G G' per-fectly free to move. These are fixed to the transversebar H H, from the centre of which proceeds the rod Ito which the vertical motion is to be given. Here it isobvious, that when the pinion A is put in rotation inthe direction of the arrow, the motion of the two wheels,and of the cranks D D , ) will have the directions shownby the arrows between B and C, which will bring theends D D' of the arms towards B and C, and thentowards IV C', and thus by the alternate closing and open-ing of the two points D D', as well as by their actuallyascending and descending, a perfectly rectilinear motionwill be given to the rod I.

(79.) Another Method. —Conceive the wheel A B, fig.

10, cut with interior teeth to be fixed in its place, withinwhich the interior wheel C D, with exterior teeth, andof half the dimensions of the fixed wheel, is made torevolve. Let a pin truly turned project from its rimexactly opposite its extreme circumference. Then shallthis point, by the revolving of the small wheel, ascend anddescend vertically in the diameter of the larger wheel,and, consequently, will give to any rod which is attachedto it the same vertical or rectilinear-motion. The resultof this operation is not so obvious as in the former case ;it may not, therefore, be amiss to give the reader a geo-metrical demonstration of the truth of the proposition.

This may be done very simply as follows.

Let oedy(fig. 10) represent the fixed wheel, andac, y c, the interior revolving wheel in two positions,being in the former at the extremity of the vertical diameterad of the larger fixed wheel, it is to be demonstrated,that when this wheel comes into any other position b' c,the point a in its circumference will always be found inthe diameter a d. Now to prove this, all that is neces-sary is to show, that the arc b' a', in the second positionintercepted between b' and the vertical ad, is alwaysequal to the arc b' a of the greater circle. Take o o'the centres of the circle in its two positions, join b' c,b o' a' o', then it is obvious, that ba — b'a', and thatthe angle b o a — angle b' o' a' — 2 b'o a, because anangle at the centre is double the angle at the circum-ference ; but the length of circular arcs are as theangles they subtend multiplied by their radii. Hence theangle of the larger circumference being half that of thesmaller, but its radius double the radius of the smaller,the arcs themselves are equal, that is, in all positionsb' a' = b a, consequently, the point a is always found inthe vertical a d.

Of the Form and Construction of Mills.

(80.) In the preceding chapter the principles of con-structing the different parts of mill work have been illus-trated; it remains for us in this to show these elementaryparts combined into the form of the mill itself, that is,we have now to explain the manner in which differentpowers are enabled to give rotation, and to act upon

m