266 Mr Sang's Annual Report on the 



■what objectionable ; the strains on the different parts of the sub- 

 stance operated on are reversed, compression being converted 

 into distension, distension into compression ; so that a source of 

 inaccuracy proportional to the flexibility of the substance is in- 

 troduced. Such extreme cases, however, seldom occur, as the 

 rigidity of each portion of a machine ought to be sufficient to 

 prevent any perceptible error of this kind. 



It is very easy to imagine other methods of cutting during 

 both motions of the plate, but Mr Whitworth's has this advan- 

 tage, that as the axis of the cylinder is adjusted perpendicular 

 to the cross slide, the tool must sink to the same depth each cut : 

 were this not the case, the surface would be reeded, the cuts 

 being alternately deep and shallow. 



The second inconvenience of the planing-engine is most appa- 

 rent when we attempt to urge it by hand. On account of the 

 great weight of the plate, immense friction is caused on its 

 slides, the pressure needed to overcome which sometimes far 

 exceeds that exerted merely in cutting. To remove this, friction- 

 rollers have been placed beneath the plate ; and although these 

 be liable to several disadvantages, such as the difficulty of accu- 

 rate lineation, and the risk of dust, planing-engines fitted in this 

 way, and carefully managed, seem to perform well. One which 

 I examined in Mr Whitworth's, produced work not at all infe- 

 rior in precision Jo that of sliding plates. Engines of this kind, 

 however, want that principle of self-correction which often ren- 

 ders a used superior to a new instrument. 



These are the two principal considerations connected with the 

 economy of the planing-engine. There are several other matters, 

 rather, however, belonging to a detailed description, than fitted 

 for a general report. These I shall pass over, excepting in so 

 far as to notice a very beautiful adaptation of the screw to the 

 driving of the engine. The plate is usually drawn backwards 

 and forwards by means of a strong chain wound on a barrel : 

 the plate of the engine which I have just mentioned was driven 

 by a screw. In order to obtain sufficient rapidity, the separa- 

 tion between the threads of the screw must be large ; were such 

 a screw to work in a common box, the friction would, on account 

 of the obliquityj be very much augmented ; to provide against 



