COH8IDKUATIOM8 



contact with iU bearing*; but when the weight is so great that the 



MOTTO on the bearing* Wcely to injure them, the greater part of the weight 



mppoctad by auxiliary bearings, the pressure of which is regulated by counter- 



niBM w gpriogs, leaving only a moderate pressure to be borne by the true 



brariflft 



8. TRANSLATION. 



Motion of translation in a fixed direction, without rotation. 



This kind of motion is required for pieces which slide along straight fixed 

 pieces, as the verniers and microscopes of measuring apparatus, such as catheto- 

 m^nrn and micrometers, the slide-rests of lathes, the pistons of steam-engines 



and pumps, Ac. 



When a tripod stand is to have a motion of this kind in a horizontal 

 plane, two of its feet may be made to slide in a V groove, while the third 

 rests on the horizontal plane. 



When a cylindrical rod is to have a longitudinal motion, it must be made 

 to bear against two fixed Y's, and must be prevented from rotating on its 

 axis by a bearing, connected either with the cylinder or the fixed piece, which 

 glides on a surface whose plane passes through the axis of the cylinder. 



When, as in cathetometers and other measuring apparatus, a piece has to 

 hliilo along a bar, the five bearings of the piece may be arranged so that 

 three of them form a triangle on one face of the bar, while the two others 

 rest against an adjacent face of the bar, the line joining these two being in 

 the direction of motion. These bearings may be kept tight, without the possi- 

 liility of jamming, by means of spring bearings against the other sides of the bar. 



9. PARALLEL MOTIOX BY LINKWORK. 



In all these methods of guiding a piece by sliding contact, there is a 

 considerable waste of energy by friction. In many cases, however, this is of 

 little moment, compared with the errors depending on the necessary imperfection 

 f the guiding surfaces, arising not only from original defects of workmanship 

 but from straining and wearing during use. 



It is true that great advances have been made, and notably by Sir J. 

 Whitworth, in the art of forming truly plane and cylindric surfaces; but even 



