MECHANICS. 



the Beater circle. Hence, the arc B E 

 is that which has rolled over B A, and 

 E is the point of the lesser circle, which 

 at the besinnins: of the motion was at 

 A, and it is now found on the line A B. 

 In the same manner it may be proved 

 that this point is found on the diameter 

 A, F in every position of the lesser 

 circle. When the lesser circle has rolled 

 over a quadrant of the greater, the 

 point is found at the centre C ; and when 

 it has rolled over a semicircle, it is found 

 at the other extremity, F, of the diameter. 

 While the lesser rolls over the other 

 semicircle, the point returns along the 

 diameter F A to A. Thus the point 

 alternately moves from A to F, and from 

 F to A. 



A very simple and obvious contri- 

 vance for producing a reciprocating 

 rectilinear motion by a continued circu- 

 lar one, is by a crank wrought by a 

 winch. A rope attached to the crank, 

 and passed over a fixed pulley, from 

 which a weight is suspended, will attain 

 the desired end. As the crank revolves, 

 the weight will evidently ascend and de- 

 scend in a straight line. 



If a connecting rod be carried from 

 the crank and connected by a joint with 

 a straight bar or beam, which moves in 

 guides, this bar will receive a recipro- 

 cating rectilinear motion. In this way 

 the piston of any species of pump may 

 be wrought by a continued circular mo- 

 tion. Such a contrivance as this is 

 used in the apparatus for condensing 

 gas, in the portable gas manufacture. 



The common apparatus, called an 

 eccentric, may be made to produce a 

 similar effect. This contrivance con- 

 sists of a circular metallic ring, the 

 inner surface of which is perfectly 

 smooth. This ring is connected by a 

 shaft and a joint, with a straight rod or 

 beam moving in guides. Within the 

 ring is fitted a circular metallic plate, 

 capable of turning freely within the 

 rins:, the surfaces in contact beins: per- 

 fectly smooth, and lubricated with oil 

 or grease. This circular plate revolves, 

 but not on its centre. It turns, in fact, 

 on an axis at some distance from its 

 centre, the effect of which is, that the 

 ring within which it turns is moved 

 alternately in opposite directions, and 

 through a space equal to twice the dis- 

 tance of the axis from the common 

 centre of the rins:, or circular plate. 

 This communicates, through the shaft 

 and joint, an alternate rectilinear motion 

 to the rod which works in the guides. 



(111.) To convert art 

 tiJhiar motion into a 

 cular on, or vice versd. 



This is one of the most useful and 

 important changes of motion which can 

 be effected by machinery, and merits 

 the greatest attention. 



A beam, having an arched head at 

 one end, bearing teeth, which work in a 

 rack, the opposite end being moved al- 

 ternately in a circular arc, will move 

 the rack alternately in a straight line. 



Also, if a weight be suspended by a 

 chain which hangs on the arched head 

 of a beam, the other end of the beam 

 bein alternately moved in a circular 

 arch" the weight will alternately ascend 

 and descend in a vertical straight line. 



An example of this will be observed 

 in the atmospheric steam-engine, and 

 of the former contrivance in the earlier 

 double-acting steam-engines of Watt. ; 



A very neat method of accomplish- 

 ing this change is represented in fig. 

 108. A B is^a lever turning on the 



centre C. D E F is a half- wheel, to 

 which, at the extremities, D F, a rope 

 or strap is attached, and is passed, as 

 represented in the figure, round the 

 wheels or sheaves MX. A reciproca- 

 ting circular motion, in the ends A B 

 of the lever, will produce a reciproca- 

 ting rectilinear motion in any point L, 

 on or connected with the rope, and vies 

 versa. 



A common bow used by watchma- 

 kers and other artists for drilling holes, 

 is another example of this. The bow- 

 string is rolled once round a small sheaf 

 or wheel carrying the piercer, and an 

 alternate rectilinear motion in the bow 

 produces an alternate circular motion 

 in the wheel. 



But by far the most remarkable, 

 method of converting an alternate rec- 

 tilinear motion into an alternate circu- 

 lar one, is that known by the name of 

 the parallel motion, and invented by 

 Watt for his double-acting steam-engine. 



In this machine the piston is urged 



