8 SCIENTIFIC APPARATUS. 



surfaces ; but even these are liable to become altered, not only by 

 wear but by strain and by inequalities of temperature, so that it 

 is never safe to depend upon the perfect accuracy of the fitting 

 of a large bearing surface, except when the pressure is very 

 great. 



In linkwork, on the other hand, the relative motion of any two 

 pieces at their mutual bearing is one of pure rotation about a 

 well-turned axle. The extent of the sliding surfaces is thus 

 reduced to a minimum,' so that less power is lost by friction, and 

 the workmanship of such bearings can be brought much nearer to 

 perfection than that of any other kind of fittings. Hence, in all 

 prime movers and other machines, in which waste of power by 

 friction is to be avoided, and even in those in which great accu- 

 racy is required, it is desirable, if it is possible, to guide the 

 motion by linkwork. 



The so-called " Parallel Motion " invented by James Watt was 

 the first attempt to guide a motion of translation by means of 

 linkwork ; but though the motion as thus guided is very nearly 

 rectilinear, it is not exactly so. Various other contrivances have 

 been invented since the time of Watt, as, for instance, that fitted 

 to the engines of the Gorgon by Mr. Seaward ; but all of them 

 involved either a deviation from true rectilinear motion, or a 

 sliding contact on a plane surface, and it was generally supposed 

 by mathematicians that a true rectilinear motion, guided by pure 

 linkwork, was a geometrical impossibility. 



It was in the year 1864 that M. Peaucellier published his 

 invention of an exact parallel motion by pure linkwork, and thus 

 opened up the path to a very great extension of the science of 

 mechanism, and its practical applications. The linkwork motions 

 constructed by M. Garcia, Mr. Penrose, and others, and the 

 extensions of the theory of linkwork by Sylvester, Hart, arid 

 Kempe, are now well known, but they could not be fully described 

 within our present limits. 



