MECHANICS. 



upwards and downwards, in a very exact 

 straight line, and any force which de- 

 flects it from this straight line will injure 

 or destroy the operation of the machine. 

 The end of the beam, on the other hand, 

 with which the piston-rod is to be con- 

 nected, moves alternately in a circular 

 arc. It became then a difficult problem 

 to connect these in such a manner that 

 a perfectly smooth motion, and free 

 from strain, should be imparted from 

 the one to the other. In the first in- 

 stance, Watt placed a rack on the end 

 of the piston-rod, which worked in teeth 

 raised upon the circular head of the 

 beam. It was found, however, that this 

 gave an uneven jolting motion, and was 

 liable to rapid wear. He soon substi- 

 tuted for it his parallel motion. 



The beam turns on the centre C, and 

 its end B moves alternately in a circu- 

 lar arc, of which C is the centre ; a rod 

 F E plays on fixed centre F, and the 

 end E moves in a circular arc, with that 

 point F as centre. The end E of the 

 rod F E is connected with a point D on 

 the arm B C by a rod which moves on 

 joints at D and E. In this arrange- 

 ment it will be observed, that the points 



D and E each move alternately in cir- 

 cular arcs. Now, it is found that the 

 middle point G of the bar D E moves 

 alternately upwards and downwards in 

 a straight line, and, therefore, a piston 

 rod attached to the point G will be 

 moved upwards and downwards in a 

 straight line*. 



Jointed at E is another rod, AE, 

 equal to B D, and another, A B, equal to 

 D E, jointed at A and B. The figure 

 A B D E is evidently a parallelogram, 

 and the point A moves in a line similar 

 and parallel to that described by G, but 



* Strictly r>peaking, the path of the point G i> not 

 a straight hue. bin i> a curve of a high order. Th-it 

 part ol tr.e curve, however, which is in.-luded within 

 the range of the pis- ton-rod lie.-, at equal di*tan<e.<i on 

 each side of a point of inflection. 'Ihe radius of 

 curvature is, therefore, innnite. and the curve 

 tUflers imperceptibly from a straight line v 



longer than it, in the proportion of B C 

 to DC. 



It is usual, in the steam-engine, to 

 attach the great piston of the cylinder 

 to the point A, and the piston "of the 

 air-pump to the point G. 



The most common proportion of the 

 several rods forming the parallel motion 

 is as follows : D is at the middle point 

 of B C ; F E is equal to D C, or half of 

 B C. Great latitude is allowed in the 

 length of D E. There are, however, 

 various proportions which may be eiven 

 to these rods, and for which we refer to 

 works on the Steam-engine. 



Another contrivance for this purpose 

 is represented in Jig. 110. A B is a 

 wheel capable of moving round an axis 

 or spindle, and also capable of moving 



Fig. no. 



longitudinally on it. A rope is attached 

 to A, and passing through a hole at the 

 top, C, of the spindle, or axis,it is fastened 

 at B, the opposite end of the diameter 

 of the wheel which passes through A. 

 By turning the wheel, the rope is 

 twisted round the spindle, and the wheel 

 drawn up towards C ; and as the rope is 

 again untwisted, the wheel descends to 

 its former position. Or, if the wheel 

 be incapable of a longitudinal motion, 

 but the spindle CD be capable of 

 moving longitudinally through it, a si- 

 milar effect will be produced. This ap 

 paratus may be used for drilling or 

 boring with the point D. 



