Nos. 18 and 19. These boilers were designed primarily for fire protection purposes, the requirements demanding a small, compact boiler with ability to raise steam quickly. These both served the purpose admirably but, as in No. 9, the only provision made for the securing of dry steam was the use of the steam dome, shown in the illustration. This dome was found inadequate and has since been abandoned in nearly all forms of boiler construction. No other remedy being suggested at the time, these boilers were not considered as desirable for general use as Nos. 21 and 22. In Europe, however, where small size units were more in demand, No. 18 was modified somewhat and used largely with excellent results. These experiments, as they may now be called, although many boilers of some of the designs were built, clearly demonstrated that the best construction and efficiency required adherence to the following elements of design: 1st. Sinuous headers for each vertical row of tubes. 2nd. A separate and independent connection with the drum, both front and rear, for each vertical row of tubes. [Illustration: No. 20A] [Illustration: No. 20B] 3rd. All joints between parts of the boiler proper to be made without bolts or screw plates. 4th. No surfaces to be used which necessitate the use of stays. 5th. The boiler supported independently of the brickwork so as to allow freedom for expansion and contraction as it is heated or cooled. 6th. Ample diameter of steam and water drums, these not to be less than 30 inches except for small size units. 7th. Every part accessible for cleaning and repairs. With these points having been determined, No. 20 was designed. This boiler had all the desirable features just enumerated, together with a number of improvements as to detail of construction. The general form of No. 15 was adhered to but the bolted connections between sections and drum and sections and mud drum were discarded in favor of connections made by short lengths of boiler tubes expanded into the adjacent parts. This boiler was suspended from girders, like No. 15, but these in turn were carried on vertical supports, leaving the pressure parts entirely free from the brickwork, the mutually deteriorating strains present where one was supported by the other being in this way overcome. Hundreds of thousands of horse power of this design were built, giving great satisfaction. The boiler was known as the "C. I. F." (cast-iron front) style, an ornamental cast-iron front having been usually furnished. [Illustration: No. 21]