862 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



while the smoke, being too great a burden for the throttled flue 

 of the tubes, is precipitated there and left ; a small proportion of 

 the heating gases entering the tubes is exhausted in the first three 

 or four feet of their length. 



The cast iron spheres of this boiler are less liable to deposits 

 of ash, and rarely so of soot. The furnace being proportionately 

 much broader and larger than that of other boilers of like capar 

 city, the circulating flues are spacious and more extensive, admit- 

 ting air freely, and ensuring almost perfect combustion of the 

 gases, which heat, in its transit to the chimney, envelopes each and 

 all the globes of the boiler. All external deposit is readily 

 removed by the steam brush used through the peep holes at any 

 time while the boiler is at work. 



Internal Deposit. 



Salt and other mineral matters held in water are, when precipi- 

 tated, less subject to adhesion in cast iron than wrought. All per- 

 sons having had experience with the cast iron heads of wrought 

 iron boilers can verify this fact. 



Mr. Colburn says of cast iron spheres in this boiler : " Although 

 the spheres may be temporarily coated internally with scale, they 

 are found to part with this whenever they are emptied of water. 

 This fact is the most striking discovery that has been made in boiler 

 making; it removes a fatal defect in small water spaces, which can 

 now be used with certainty of their remaining clear of deposit." 



This, I think, may be accounted for. The process of precipita- 

 tion of mineral matters in all boilers being from like causes the 

 same. Those foreign properties in the water concentrate and pre- 

 cipitate from its evaporation, and particularly being more soluble 

 in hot than cold water, settle on the slightest depression of the 

 temperature, and the lower the heat the greater the deposit. The 

 heat being again increased, the stratum is indurated, and once this 

 germ of evil is formed in wrought iron boilers, adhesion being 

 fixed, accumulation goes on rapidly, increasing the danger from 

 day to day. 



With the cast-iron globes this action is the same as with 

 wrought-iron boilers, but the scale not adhering to cast-iron as to 

 wrought. When the globes are next heated after the scale is 

 formed, the sphere, expanding in all directions, however slightly, 

 jiarts from the indurated shell, admitting an infinite film of water 

 between the shell and the iron; the water then bursting into steam 



