320 



raoi'e than 15 fts. j^er square inch, and the density of water about 16 ozs. per 

 gallon ;• — this is about double the density used in many marine boilers. 

 During the latter two weeks, the pressure was maintained at 30 lbs. per square 

 inch, and the density at 21 to 23 ozs. to the gallon, or what is commonly 



4 



expressed as ^ , that ratio expressing the weight of solid matter to that of the 

 water. Ordinary marine boilers could not be worked many days at this 

 density without being totally ruined, and a very few hours would suffice to 

 allow a thick scale to be formed all over the heating surface. Yery few of our 



2 



coasting steamers are worked with water over ^, or about 10 ozs. to the 

 gallon. About 200 gallons of sea water were evaporated, and thus over 

 1000 ozs. of solid matter were passed through the tubes. Under all the 

 circumstances, it was never expected to find the tubes entirely free from scale, 

 yet, on examination, they were found to be so, with the exception of one part 

 of each ; and this exception may be looked upon as the most valuable part of 

 the results, showing clearly, as it did, that it was to circulation of water alone 

 that the perfectly clean state of the tubes could be attributed. In this 

 instance, the inner tube instead of being suspended clear of the bottom of the 

 outer one is resting on it, and the passage of water provided for by two 

 apertures on opposite sides. Hence the currents of water at the lower end 

 were confined to the parts opposite these holes. On the other parts, a solid 

 brown incrustation of salt and lime had formed, sufficient to cement the 

 tubes together, and requiring some little force to separate them. The scale 

 was rather less than one-eighth of an inch thick, and extended about one inch 

 up the blank ends of the inner tubes. 



To all who witnessed the exj^eriment, no result could have been more 

 conclusive other than that of actual trial for a considerable period at sea, and 

 the writer is convinced that, with well-arranged proportions, the " Field " 

 boiler can be worked at sea with at least equal facility with the best design of 

 common tubular boilers, and certainly with veiy much more ease and safety 

 than wnth numbers of those in common use. 



The experience of the writer leads to the following as the principal points 

 to be attended to in designing " Field " boilers. The tubes should be long and 

 few, rather than short and numerous ; the lower ends should be hemispherical, 

 and not flat ; the upper ends of the inner tubes should be carefully opened out 

 to a circular curve (more correctly a parabolic one), instead of the flat funnel 

 mouth mostly used ; and the flame and hot currents should be made to 

 circulate perfectly around the pendant tubes. 



It is found to be of the very greatest importance that water free from 

 vegetable matter should be used in these boilers, as the slightest concentration 

 of such causes, with the rapid circulation, determined cases of priming, and has 

 been known to choke the inner tubes when allowed to go too far. 



