Have I Got News For You on BBC featured the Chimney Journal as its guest publication recently, in which the Carlisle Blow Down Pot, made by W T Knowles of Elland, was mentioned in a missing headline. This chimneypot is a simple clay shaft with a 'can top' placed on protruding lugs, with a vent gap between the can and the shaft theoretically allowing increased draft in turbulent winds.
 
The great cottage architect John Claudius Loudon wrote: Many of the designs for chimney-tops and pots which occur … appear to me liable to objection from their being so massive at the orifice of the summit that the wind will be thrown into eddies by them; which in chimneys of uncertain draught may interfere with the free discharge of the smoke.
 
I apprehend that whatever may be the form of the termination which may be adopted for chimney-tops the last portion of them should approach, as nearly as can be, to a thin tube, which may interfere as little as possible with the flow of the external air, which is to carry away the smoke laterally …
 
… the great majority of chimney-pots, in every part of Britain, are formed of thin earthenware tubes … let a chimney be built and terminated as it will, very little can be affirmed, by the builder or architect till it has been tried as to whether it will draw or not.
An Encyclopaedia of Cottage Farm and Villa Architecture, published in 1842.
 
A single or group traditional hand thrown roll top or cannon head pot will create a smooth Venturi effect. A bishop or crown pot will create eddies and disrupt the Venturi effect in lighter winds, although these may be reduced by the addition of upward flow louvre vents or other such gismos.
 
The Carlisle Blow Down by Knowles, established in 1906, is a can top downdraft preventer.