ON PRESSURE AND EQUILIBRIUM. 67 



same manner, it is necessary that tlie platten of a printing press, or the part 

 which presses the pap^^i'- on the types, should descend from a considerable 

 height, blrt'jft is Only at; the imtant of taking off the impression that a great 

 force is required; and both these' ca'ds are obtained by similar means in a press 

 lately invented by Lord Stanhope. (Plate III. Fig. 48,. 49.) 



The wheel and axis bear a very strong resemblance to the lever. . i|",.twd 

 threads, or perfectly flexible and inextensible lines, be wound- in contrary di- 

 rections round two cylinders, drums, or rollers, moveable tog-ether on ".the 

 $ame axis, there will be an eciuilibrium, when the weights attached to thft 

 threads, or- the forces operating on tlienij- are inversely as the radii of the cy- 

 linders, or as the diameters of which they are the halves. It may easily be 

 understood, that the weights have the same power in turning round the cy- 

 linders, as if they were immediately attached to the arms of a lever, equal in 

 length to their semidiameter, and that the conditions of equilibrium will be 

 the same. The demonstration may also be more immediately deduced from 

 the position of the centre of gravity, immediately below the axis of the cy- 

 linders, which requires the weights to be inversely as the radii. With respect 

 to stability, the equilibrium is neutral, and the cylinders will remain at rest 

 in any situation. A single cylinder is also often combined with a levea- or 

 winch, and in this case the radius of tire cy.linder is to be compared with the 

 Jeagth-of the lever ipr;wi<ieh. (Plate III. Fig. 50.) 



Systems of wheels and pinions, of various kinds, resemble, in their mecha- 

 nical properties, either a series of levers, or the combination of cylinders, 

 which constitutes the wheel and axis; but the form of the teeth may produce 

 a difference in their action, which will be mentioned when the practical con- 

 struction of wheelwork is discussed. 



Sometimes the axis connected with a winch is composed of two cylinders, 

 one end of the rope being uncoiled from the smaller, while the other end 

 winds round the larger ; the weight being supported by a pulley rimning in 

 its angle. Here the conditions of equilibrium are easily determined from the 

 place of the centre of gravity, and the effect of the machine is the same, as if 

 the weight* were attached to a rope coiled round a simple cylinder, of a dia- 

 meter equal to half the difference of the diameters of the double axis. The 



