. Defcription of a new Venlilator. f 



CC is an external tube, clofely foldered or cemented to the fixed tube A, fo that, w4tb 

 its upper part, it may form a deep groove of four or five inches, in which the lower extre- 

 mity of the moveable tube may freely revolve : this groove fliould be filled with mercury, 

 high enough to cover the bottom of the moveable tube about half an inch ; which con- 

 trivance willeiFedually prevent the outer air from being drawn in between the tubes, with- 

 out impeding the motion of the upper one in the leaft. F F is a fmall pro;e£lion faftened 

 to the moveable tube, to prevent the rain or duft from falling into the groove. 



The Blaft Ventilator would be particularly ferviceable to fliip chimnies above all others. 

 Any one who has ever been at fea, muft have obferved the difliculty of preventing (hip- 

 chimnies from fmoking when the wind rifes to any height, and what an advantage any 

 contrivance would be that would prevent this inconvenience j and that this inftrument 

 would be quite efFe£lual for this purpofe, (there is every reafon to believe from the expe- 

 riments already made) and even make thofe chimnies draw better as the wind blew 

 harder. 



The inftrument reprefented in the two firft figures would be the fitteft for this purpofe, 

 any of the men on deck could eafily fhift it about as the wind changed ; and as this way of 

 ufing it might be made for a few {hillings, the expence will be no obje£i to prevent (hip- 

 owners from giving it a fair trial. 



It is fcarcely neceflary to mention, that it would do equally well for the fame purpofe in 

 canal pafTage-boats, which are often as much incommoded by fmoke as fliips are ; a mode 

 of conveyance to which in fome fituations, elegance is added to eafe and cheapnefs, and 

 which well deferves farther extenfion on the Englilh canals. 



The manner in which the wind operates in this apparatus, to caufe a current of air 

 through the upright tube, is moft probably the following : 



When the conical opening C D, Fig. 2, is turned to the wind, the portion of the cur- 

 rent of air which enters this opening, will be compreffed againft the fides of the cone, as it 

 pafles on, more and more, till it comes to E ; and after it has pafibd F, the extremity of the 

 fmall tube, it will begin to expand itfelf ; but as it moves forward while thus expandings 

 the union of the two motions will caufe it to form the figure of a cone or conoid, fomewhat 

 fimilar to that by which it entered, which is marked by the dotted lines terminating at 

 GandH. 



This conical blaft ftriking obliquely the air before it in the tube B, will caufe it to move 

 forward only, its lateral motion being prevented by the fides of the tube ; and the preflure 

 of the atmofphere forcing the air forward at A, to replace what has been removed at B by 

 the conical blaft, will of courfe caufe the current of air from A through B already men- 

 tioned. 



In fituations where coals may be had cheap, fteam may be made to operate on the blaft 

 ventilator inftead of wind, for many ufeful purpofes, by a contrivance fomewhat fimilar to 

 the eolipile ; with fome alteration as to its form, but preferving the fame principle ; par- 

 ticularly fteam might be ufed with this inftrument fo as to force air. on a blaft furnace, and. 



would 



