142 ACTION OF THE FLAME.

the greater breadth is at the top of the chamber,—this, doubtless, is as much to preserve accessfor repair as notion of design.

The arrangement of the tubes can be betterunderstood by referring to the sectional eleva-tion. It is seen that above each fire box arearranged a given number of tubes in the ordi-nary manner. To reduce the total height of theshell, the remainder of the tubes are arrangedat the side of the inner fire box; the top rowof the entire set being in a straight line, andthus one level is preserved. The level of thebottom row of the side cluster is at about thatof the fire grates.

The smoke box partakes also of the novelty oidesign common to the remaining portions, due,of course, to the arrangement of the tubes.

Apart from the tubes being so disposed toreduce the height of the shell, the spacebetween the roof and the water line is alsocontracted to a minimum, being about seven oreight inches. Now it is evident that thisheight is inadequate for the steam roomrequisite.

The surmounting of difficulties is always atriumph in all grades of pursuit, and, often,none are more worthy of congratulation thanthe designers of marine engines and boilers.It often occurs, however, that the gain certainin one locality causes an imperfection inanother; it is therefore obvious that theacquisition of desiderata common to the wholearrangement is no ordinary attainment. Howfar these remarks apply to the example of lowboiler under notice needs no comment for thepresent purpose. It is sufficient to recognizefacts without personal opinions.

The water line in the boiler now under

direct notice is stated to be so near the roof,that a separate steam space is requisite, whichis attained by a compartment being formed atthe end of the shell, the communication forthe traverse of the steam being at the roof.It is seen also that the steam space has a casingsurrounding it, separate from the water com-partment in the boiler. At the base line apipe is secured for the purpose of dischargingany water or condensed steam that mayaccumulate within the casing in question.

The arrangement under notice is compactin the full meaning of the word, relative to thevertical positions of the details especially. Itmust be noticed, however, that the correct totalcubical capacity, proportionately to the area ofthe grate surface, is unalterable with all boilers.The space that is generally above the tubeswith other arrangements, is at the side or endof the shell in this example. Room in thehull is therefore required, longitudinally ratherthan in any other direction.

Having briefly commented on the arrange-ment in question, allusion will next be givento the action of the flame. In the presentexample, two lines of progression are inoperation at the same time: the one at rightangles vertically, and the other horizontallywith the furnaces. These variations are, ofcourse, due to the position of the tubes.

The flame, on entering the combustion cham-ber, has to divide itself into two volumes, onepertaining to the tubes directly over the fireboxes, and the other to the tubes at the side.Now the situation of the uptake—as beforenoticed—to a great extent governs the actionof the flame.

With the present arrangement, the fire boxes