first instance, and vastly more advantageous in all respects, than any plain cylindrical furnace, by reason of its greatly less weight in proportion to its resistance of a given compressive strain, by reason of its greater economy of fuel due to the nearer approach to perfect combustion which it causes, and by reason of its greater facility in handling, cleaning, etc. The weight for equal resistance to the same external pressure, of the corrugated cylindrical furnace is not one-fourth of the weight of a plain cylindrical furnace of the same diameter and length.

It should be remembered that twenty years ago steam pressure in marine practice reached 30 and 40 pounds to the square inch, being considered high at the latter figure; while, with the triple and quadruple expansion engines, pressure of from 160 to 180 pounds are demanded.

Montgomery's claim in patenting his idea of corrugated cylinders was primarily for the structural strength combined with material lightness resultant. We are not certain whether or not he anticipated or seriously appreciated the more perfect combustion that his corrugations would Continued experience has demonstrated the scientific soundness of his assumptions as to strength, the processes which are used neccessitating the best quality of metal, the operation giving to the finished work a perfectly cylindrical shaping impracticable in any other mode of

assure.

production as a rule, and the pressure of corrugation not only effecting a perfect uniformity of all parts, preserving an exact standard of thickness, and perfectly graduating the angles of contour, but subjecting the material to a test which would infallibly discover any molecular defect. The natural consequence of all this is that the furnace, formerly the weakest and most sensitive feature of the boiler, has become the strongest, thereby contributing greatly to the durable character of the boiler.

The better combustion obtainable with the corrugated furnaces is a direct result of the corrugations. They thoroughly mix mechanically the current of the gases of combustion which rise from the fuel on the grate in an unmixed state, the combustible gases being in separate masses or streaks side by side with the air whose oxygen is needed for their combustion. The entire mass of gas and air when at its highest temperature should be completely mixed for perfect combustion, and the corrugations act to that effect as "riffles on the gaseous current passing across them, so that at the upper time and place the mixing of the fuel gases with the air is mechanically accomplished with mechanical certainty. If the mixing be performed by any apparatus-special or otherwise-beyond the furnace, when the temperature of the gases will have unavoidably fallen, the economic effect will

be comparatively slight; it must be done in the furnace for maximum results, and no means have been devised so simple and efficacious as the corrugations, which perform it merely as a consequence of their form and location.

FIG. 57.-T. F. ROWLAND & Co.'s FURNACE. (AMERICAN.)

FIG. 58.-PURVES' FURNACE. (ENGLISH.)

24

FIG. 59.-HOLMES' FURNACE. (ENGLISH.)

FIG. 60.-FARNLEY FURNACE. (ENGLISH.)

FIG. 61.-FOX'S FURNACE. (ENGLISH.)

CHAPTER XIX.

RIVETED SEAMS.

"THE pressure for any dimensions of boilers not found in the table annexed to these rules, must be ascertained by the following rule, viz: Multiply one-sixth () of the lowest tensile strength found stamped on any plate in the cylindrical shell by the thickness-expressed in inches or parts of an inch-of the thinnest plate in the same cylindrical shell, and divide by the radius or half diameter-also expressed in inches-and the sum will be the pressure allowable per square inch of surface for single riveting, to which add twenty per centum for double riveting."

So runs the beautifully simple rule prescribed by our Board of Supervising Inspectors of Steam Vessels for ascertaining the pressure allowable for dimensions of boilers.

Some two or three years ago we were engaged designing a marine boiler to have as thin a shell as possible consistent with safety for a lightdraft steamer, and great was our surprise when informed that, though we made the longitudinal seams triple-riveted according to the most approved practice, we would not be allowed any

* Oldham.