the common Turning Lathe. • 175 



to be turned, at the commencement of the process, and 

 to become parallel to it when the screw was finished, an 

 approximation would take place from a larger screw to a 

 lesser, or vice versa ; but it is impossible that the rest could 

 become parallel to the work, from the connection of the 

 arms. Now let the traversing arm, D, lie in the centre of 

 the screw B, on which it plays, and let the rest make a 

 right angle with the wood on which we intend to cut the 

 screw. The rest may traverse thirty degrees on either side 

 of the right angle; which will not cause any sensible ap- 

 proximation in the thread, and will admit a motion suf- 

 ficiently extensive for turning the common length of screws. 

 But as the method answers for a short screw of a hw turns, 

 that is sufficient for every purpose. For, in order to make 

 a long screw, there may be three different ways for doing 

 it: — 1st, At the commencement the rest stands at right 

 angles with the wood on which the screw is to be cut ; by 

 it describing an arch of a few degrees, a short screw is cut ; 

 then by bringing back the rest to its original angle, the right 

 one, and sliding forward the single-pointed tool to the last 

 thread of the screw that was just cut, we proceed to any 

 length by repeating the same process, ^dly, When one or 

 two threads of a screw is cut, by making use of a common 

 screw tool, the most unskilful hand will be able to continue 

 the screw to any length. 3dly, Should a side tool with 

 jnany teeth be made use of, a screw of any length may be 

 cut, the rest describing its usual arch. But actual expe- 

 rience of the utility will, I trust, also do away all objection; 

 for I assure you, when I first thought of the method, I re- 

 duced it to practice, and, having made the instrument, 

 worked with it, to my utmost satisfaction, in the presence 

 of many friends. Permit me to return you my many thanks 

 for the trouble you have taken in considering this paper, 

 and also for the friendly manner in which you expressed 

 your objection. Believe me, sir, 



Your much obliged servant, 



Robert He alt. 



LETTER 



