State of the Useful Arts. 265 



The motions, then, of all machines, are performed by help 

 of slides, axes, or screws ; and thus the consideration of the arts 

 of formation of machines resolves itself into three principal 

 heads ; the first treating of the planing-engine, the second of 

 the turning-lathe, and the third of the screwing-engine. 



The planing-engine is now so generally known, and so uni- 

 versally employed, that I might be excused by most of those 

 who take an interest in the proceedings of such a Society as 

 this, from giving a description of its general action. It may be 

 well, however, to glance at the nature of the process. 



The planing-engine is used for making straight and flat 

 work ; for this purpose a large and heavy plate is fitted to slide 

 accurately upon a frame. Over the path of that plate there is 

 fixed a second slide carrying the cutting tool. The material to 

 be operated on is secured on the moving plate, so that when the 

 plate is drawn along, the cutter removes part of the substance, 

 making a straight cut; the sliding plate is then returned, pro- 

 vision being made to raise the tool during this motion, and a 

 second cut is made parallel to, and close by the first ; the tool 

 having, previously to the renewal of the motion, been displaced 

 a little by its regulating screw. Such is the general action of 

 the machine ; it has its inconveniencies. 



The most serious inconvenience, that which is not merely no- 

 ticed, but felt, by a person watching its performance, is, that 

 the whole time of the back-motion is lost; and, along with the 

 time, a considerable amount of" power. This inconvenience has 

 been removed by an ingenious contrivance of Mr Whitworth of 

 Manchester. He places the tool in a cylindric holder, and thus 

 provides for its being turned round. As soon as a cut has been 

 made in one direction, and while the action is being reversed, 

 this cylinder, and along with it the tool, is turned half round, so 

 that a second cut is made while the plate is returning. In this 

 way, by the reversion of the tool, at each i-eversion of the plate, 

 the cutting goes on without interruption. 



For work not requiring the utmost degree of precision, this 

 back-cutting must undoubtedly be of immense advantage ; but 

 where very great delicacy is needed, the method seems some- 



