134 LECTURES TO SCIENCE TEACHERS. 



another set of circumstances to be considered, namely, that 

 when cylindrical flues are exposed to external pressure and 

 I think we owe it to the late Sir William Fairbairn that 

 that which was formerly a most fruitful source of giving way 

 in steam-boilers has been cured, because, although a perfect 

 cylinder subjected to pressure ought to bear more than it 

 would now, the pressure applied as an internal bursting strain, 

 yet even a perfect cylinder when pressed from the outside is 

 in a state of unstable equilibrium, and, in practice, a flue thus 

 pressed, therefore, is subject to being destroyed at almost 

 any moment, either by its not having been originally a 

 perfect cylinder, or by differences of temperature, or by 

 wear ; and we know as a fact that such flues collapsed 

 under pressure, and thus they were a constant source of 

 accident in internally fired boilers. Now this defect has been 

 entirely cured by the expedient of riveting at intervals T 

 iron, or a specially made sort of channel iron, round about 

 the flues, but there is one very simple mode applicable to 

 all existing boilers which I think is equally efficacious with 

 the riveting system, and is better in other respects, as there 

 are no rivets to be acted upon by the fire. That mode 

 consists in simply cutting out of a piece of boiler-plate a 

 ring (which if it be in an existing boiler may be made in 

 halves and bolted together), then if that ring be slid on over 

 the tube so as to fit it, the edge of the ring where it touches 

 being bevelled off, in order that there may be as little a part 

 of the tube cut off from water contact as possible, it will be 

 found, although there is no attachment between the ring and 

 the tube, that you have most effectually prevented the tube 

 from collapsing, for the simple reason that it cannot diminish 

 its diameter in one direction without correspondingly increas- 

 ing it in another. In that way without any rivet or attachment, 

 it is possible to make flues of Lancashire and Cornish boilers 

 safe against collapsing. 



The only other kind of boiler I will mention to you is one 

 which was in use a great many years ago, and was called the 

 " Quicksilver-boiler." It was employed on a large scale (that 

 is to say on a fair working scale), to drive the mill engine of 

 the King and Queen Ironworks, the property of Mr. Howard, 

 the inventor, also on a paddle-steamer called the " Vesta " 

 which ran as a trading boat to Ramsgate. This boiler had 

 a double bottom to it. The upper one (the false bottom) 





