GENERAL CONSIDERATIONS. 7 



of which is regulated by counterpoises or springs, leaving only a 

 moderate Dressure to be borne by the true bearings. 



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 cathetometers and micrometers, the slide-rests 

 of lathes, the pistons of steam engines and pumps, &c. 



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

 zontal 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 slides on a surface whose plane 

 passes through the axis of the cylinder. 



When, as in cathetometers and other measuring apparatus, a 

 piece has to slide along a bar, the five bearings of the piece maybe 

 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 possibility of jamming, 

 by means of spring bearings against the other sides of the bar. 



9. PARALLEL MOTION 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 de- 

 pending on the necessary imperfection of the guiding surfaces, 

 arising not only from original defects of workmanship but from 

 t raining 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 



