MAKING SECTIONS OF WOOD, ETC. 



337 



the screw has forty threads in the inch, and its head is divided 

 into twenty-five parts, it follows that each turn of the screw 

 will raise the cylinder one-fortieth of an inch, and each frac- 

 tion of a turn the one-thousandth of the same quantity. This 

 machine has very many advantages ; these consist principally 

 in the mode in which the knife is fixed, and also in the plan 

 of the wood about to be cut being firmly supported on all 

 sides by metal, but in such a manner as to keep the latter 

 without the reach of the knife, the screw being so short as not 

 to be able to raise the cylinder quite as high as its edge. In 

 most machines of this kind the knife rubs upon the brass, by 

 which the cutting edge is liable to injury, and the wood is not 

 driven tightly into a cyhnder, but is raised out of it by the 

 screw, consequently it cannot be kept so firmly against the 

 cutting edge, which will be found very inconvenient for hair 

 and such other soft structures as require to be securely wedged 

 up before sections of them can be made. 



Mr. Topping has contrived a very convenient and useful 

 form of cutting-machine on a plan represented in section in 

 fig. 225. A B is a flat piece of mahogany, seven inches long. 



Fig. 225, 



22 



