The economical performance of these boilers with a similar ratio of heating to grate surface is equal to that of other kinds. In fact, they are usually given a somewhat higher ratio, and their economy of fuel frequently exceeds that of the other types. Their principal defect is their small capacity for steam and water, which makes it extremely difficult to obtain steady steam-pressure. Where they are employed, the feed and draught should be, if possible, controlled by automatic attachments, and the feed-water heated to the highest attainable temperature. Their satisfactory working depends, more than in other cases, on the ability of the fireman, and can only be secured by the exercise of both care and skill. Many forms of these boilers have been devised. Walter Hancock constructed boilers for his steam-carriage of flat plates connected by stay-bolts, several such sections composing the boiler; and about the same time (1828) Sir Goldsworthy Gurney constructed for a similar purpose boilers consisting of a steam and a water reservoir, placed one above the other, and connected by triangularly-bent water-tubes exposed to the heat of the furnace-gases. Jacob Perkins made many experiments looking to the employment of very high steam-pressures, and in 1831 patented a boiler of this class, in which the heating-surfaces nearest the fire were composed of iron tubes, which tubes also served as grate-bars. The steam and water space was principally comprised within a comparatively large chamber, of which the walls were secured by closely distributed stay-bolts. For extremely high pressures, boilers composed only of tubes were used. Dr. Ernst Alban described the boiler already referred to, and its construction and operation, and stated that he had experimented with pressures as high as 1,000 pounds to the square inch. The Harrison steam-boiler, which has been many years in use in the United States, consists of several sections, each of which is made up of hollow globes of cast-iron, communicating with each other by necks cast upon the spheres, and fitted together with faced joints. Long bolts, extending from end to end of each row, bind the spheres together. (_See_ Fig. 111.) [Illustration: FIG. 111.--Harrison's Sectional Boiler.]