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extent at all were formed within tlie tubes, they could not be used without 

 risk and trouble far outweighing their advantages. Consideration of the fact 

 that the "Field" tubes owe their value solely to an ingenious device for pro- 

 moting to the utmost the circulation of the water, and also, seeing that 

 wherever in common boilers circulation is more than ordinai'ily active, very 

 little, and sometimes no incrustation with salt or lime takes j^lace, there seemed 

 good reason to expect that the new tubes could also be worked at sea. But 

 under what cii'cumstances 1 with what density of water 1 and whether or not 

 any scale at all was formed, no specific information could be gathered. The 

 writer then determined to institute an experiment, which, with the co-operation 

 of Mr. R. H. Yeoman, Boiler Maker, Auckland, he was enabled to carry to a 

 satisfactory conclusion, and of which the following is a brief record. 



As the principle of the " Field " tubes is probably unknown to many of 

 those unacquainted with steam power, its action may be shortly described, if we 

 conceive a tube open at the upper end, and closed at the lower, passing through, 

 and suspended from, the crown of the furnace or flue of any boiler. An inner 

 tube, 02:>en at both ends, and about half the diameter of the outer one, is sus- 

 pended and steadied by small ribs or feathers, so as to be clear of the bottom 

 about twice its own diameter, and project a little way at the top, above the 

 tube plate, but terminating beloio the loater level; it is "bell-mouthed" at the 

 iipper end. 



On the application of heat, the water in the annular space betAveen the 

 tubes is the first to be affected, and at once begins to rise, its place being 

 taken by the cooler water from above, descending through the inner tube. This 

 circulation, as the heat is continued goes on increasing, until, on the formation 

 of steam in the anniilar space, it is enormously increased, owing to the very 

 great difference in specific gi-avity between steam, and water even at the 

 boiling point. 



The experimental boiler was originally intended by Mr. Yeoman to show 

 visibly the eftect of the circulating tubes, which it very clearly did, being, for this 

 purpose, at first uncovered at the upper end. It had seven tubes, about 2 inches 

 diameter and about 1 foot 6 inches long ; internal tubes 1 inch diameter, but 

 their funnel mouths were not of the most effectual shape. For the jiurposes 

 of experiment, this boiler was covered in, and fitted with blow-off cock, gauge 

 glass, and safety valve, and the water was supplied by a force pump. 



The duration of the experiment was three weeks, the boiler being 

 regularly attented during working hours ; and the pressxire of steam and 

 density of water kept nearly uniform to the extent required. The salinometer, 

 by which the density of water was regulated, was first carefully tested for 

 accuracy, and the amount of solid matter in the sea water used ascertained by 

 evaporation. This was found to be 5|- ozs. per gallon. 



During the first week of the experiment, the pressure was not worked at 



