THE SMOKE- JACK. 



499 



vessels. We will now extend it a little further, and see how 

 it is modified so as to perform other offices than those which 

 have been described. 



Allusion has already been made to the Spiral or Wedge 

 principle, but some of the illustrations were accidentally 

 omitted. I therefore introduce them here, this being a chapter 

 of miscellanea. 



The Windmill has previously been described, as has also the 

 ship's Screw, another form of which is here given. 



In the centre is shown the mechanism popularly known as 

 the Smoke- jack, though it really works by means of hot air, 



birds' wings and tails. 



SMOKE-JACK. SHIP'S SCBEW. WINDMILL. KITS. 



and only becomes gradually choked by the soot which the 

 smoke by degrees deposits upon it. It is, in fact, nothing but 

 a windmill working horizontally instead of vertically, the vanes 

 being moved by the rapidly ascending heated air. So powerful 

 is the spiral pressure of this air, that in my old college days 

 at least a dozen rows of heavily laden spits were perpetually 

 turned by a single Smoke-jack. It is many years since I 

 visited my old college, and I cannot say whether the Smoke- 

 jack still exists, but, as it did its work well so long ago, I 

 presume that it does so now. 



Then there is the well-known spiral ventilator set in the 



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