CONCERNING SCIENTIFIC APPARATUS. 511 



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 accuracy 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, and Kempe, are 

 now well known, but they could not be fully described within our present limits. 



10. SCREW MOTION. 



The adjustments of instruments are to a great extent made by means of 

 screws. In the case of levelling screws, which bear the weight of an instru- 

 ment, the thread of the screw is always in contact with its proper bearing 

 in the nut; but in micrometer screws it is necessary to secure this contact 

 by means of a spring. This spring is sometimes made to bear against the end 

 of the screw, or a shoulder turned upon it; but this arrangement causes a 



