3 2 Tra nsactions. — Miscellaneous. 



under double the pressure of a cold blast. Froai tlie increased diffusibility of 

 these gases, it follows that the boiler tubes can be reduced in diameter, and 

 consequently made thinner with safety ; and, as combustion is rendered 

 complete, there will be no smoke to deposit soot in them. 



The advantage to be derived from the use of a hot, in place of a cold, blast 

 is clearly proved in a series of blow-pipe experiments made at the laboratory 

 of the Geological Survey of New Zealand, and published in Vol. II. of the 

 " Transactions," where it is shown that such refractory substances as platinum, 

 fire-clay, flint, pipe-clay, agate, and opal, were fusible if air at a temperature 

 of 500° F. be emplo3^ed. And with an exalted intensity of heat in the 

 furnace, we are enabled to avail ourselves to a still greater extent of the 

 economy arising from both the super-heating of the steam and its subsequent 

 expansion in the cylinders of the steam engine. 



Description of Plate. 



A Tlie air tube, tlie top of wMcli supplies the blower E. 



B The tube containing the gases of combustion. 



G Ash pan, containing water. 



D A door which can drop and close the tube B. 



G A tube, or space, the full width of the thermo -con vector, through which the waste 

 and cooled gases can be discharged upwards to the funnel. 



B The blowing apparatus supplied with the fresh heated air from the top of the 

 tube A, by means of a space similar to G, and which fills in the front of the convector, 

 but is removed in the figure hi order that the interior can be seen. 



H Reservoir of hot air. 



J Water space of boiler perforated by the air tubes. 



K Fire doors, enclosing space holding a charge of fuel. 



Art. V. — On the most Economic Mode of Felting Steam Boilers. 

 By J. C. Firth. 



(PL v., figs. 1 and 2.) 

 [Read before the Auckland Institute, 6th October, 1873.] 

 During the period when low-pressui-e condensing steam engines were in 

 general use, various plans were adopted to prevent waste of steam and heat in 

 boilei's by condensation or radiation. With boilers at lOlbs. to 151bs. pressure, 

 a simple covering of felt, protected by wood or canvas, answered sufficiently 

 well. But when steam boilers are run, as at present, at 501bs., 751bs., 

 or lOOIbs. per inch, for working steam expansively, or for working 

 compound engines, it has become much more important that the surface of a 



