The next step, and the one which connects the boilers as described above to the boiler as it is built to-day, was the design illustrated in No. 21. These boilers were known as the "W. I. F." style, the fronts furnished as part of the equipment being constructed largely of wrought iron. The cast-iron drumheads used in No. 20 were replaced by wrought-steel flanged and "bumped" heads. The drums were made longer and the sections connected to wrought-steel cross boxes riveted to the bottom of the drums. The boilers were supported by girders and columns as in No. 20. [Illustration: No. 22] No. 22. This boiler, which is designated as the "Vertical Header" type, has the same general features of construction as No. 21, except that the tube sheet side of the headers is "stepped" to allow the headers to be placed vertically and at right angles to the drum and still maintain the tubes at the angle used in Nos. 20 and 21. [Illustration: No. 23] No. 23, or the cross drum design of boiler, is a development of the Babcock & Wilcox marine boiler, in which the cross drum is used exclusively. The experience of the Glasgow Works of The Babcock & Wilcox, Ltd., with No. 18 proved that proper attention to details of construction would make it a most desirable form of boiler where headroom was limited. A large number of this design have been successfully installed and are giving satisfactory results under widely varying conditions. The cross drum boiler is also built in a vertical header design. Boilers Nos. 21, 22 and 23, with a few modifications, are now the standard forms. These designs are illustrated, as they are constructed to-day, on pages 48, 52, 54, 58 and 60. The last step in the development of the water-tube boiler, beyond which it seems almost impossible for science and skill to advance, consists in the making of all pressure parts of the boiler of wrought steel, including sinuous headers, cross boxes, nozzles, and the like. This construction was the result of the demands of certain Continental laws that are coming into general vogue in this country. The Babcock & Wilcox Co. have at the present time a plant producing steel forgings that have been pronounced by the _London Engineer_ to be "a perfect triumph of the forgers' art". The various designs of this all wrought-steel boiler are fully illustrated in the following pages. [Illustration: Wrought-steel Vertical Header Longitudinal Drum Babcock & Wilcox Boiler, Equipped with Babcock & Wilcox Superheater and Babcock & Wilcox Chain Grate Stoker] THE BABCOCK & WILCOX BOILER The following brief description of the Babcock & Wilcox boiler will clearly indicate the manner in which it fulfills the requirements of the perfect steam boiler already enumerated.