) 72 On cutting Screws ly Means of 



He also makes a distinction between integrant particles 

 and sulstractive particles ; these latter are always parallelo- 

 pipeclons. I shall speedily mention whence they derive 

 their name. They are substituted for the integrant particles 

 to facilitate calculations ; and it is worthy of observation, 

 that parallelopipedons can always be obtained in all dissec- 

 tions of crystals. 



Thus far, sir, I have stated the first principles of mine- 

 lalogical analysis 5 I shall now proceed to the synthesis. 



[To be continued.] 



XXX. On cutting Screws ly Means of the common Turning 

 Lathe. By Robert Healy, A, B. 



To Mr. Tilloch. 



Dublin, Dec. z, 1803, 

 . SIR, James-street, 43. 



An application which I propose for adapting the plain 

 lathe to the purpose of cutting screws is submitted to your 

 consideration, which if you deem worthy of insertion, your 

 doing so will oblige 



Your obedient servant, 



Robert Healy. 



Turning, like many of the other arts, unites the agreea- 

 ble with the useful, and, whilst it is a necessary helpmate 

 to several branches of mechanics, it often induces gentlemen, 

 by the ease with which it is accomplished, to fill up their 

 vacant hours in forming a variety of things the execution 

 of which both pleases the fancy and exercises the ingenuity* 

 But to expatiate here on its advantages is not my intention : 

 suffice it to say, that of the different branches of the art 

 that of turning the screw has been most cultivated, not only 

 from the utility which arises from it, but perhaps from the 

 celebrity that is pained by it, since he is looked upon as 

 having obtained the summit of perfection in his art, who 

 can cut every variety of the screw with facility. Thus, 

 then, it was necessary for those who could not afford time 

 sufficient for this acquirement, either to resign all idea of 

 turning them, or else oear with the inconveniences of doing- 

 it imperfectly, till the traversing manderil was thought of; 

 and from its general use among one class of turners, it is 

 obvious that it affords an easy, pleasing, and certain method 

 of turning screws. But to those who require most its as- 

 sistance 



