high temperature. When the heavy demand for light arises in the evening, steam is drawn from these reservoirs. The losses of heat by radiation from the reservoirs can be made small by covering them with non-conducting material. Mr. Halpin claims that he can replace 22 boilers, working in the ordinary way, by 5 boilers and 92 of his storage cylinders, which are cheap to construct, and have a much smaller depreciation than the boilers. The advantages of the thermal-storage system would be that the wear and tear and waste of fuel involved in firing up a number of boilers for a few hours' work is avoided. The objection to the plan is that it is peculiar, involves the use of two different kinds of apparatus, and has not been tried sufficiently to demonstrate that it will work successfully in actual practice. The various kinds of boilers may be classified as follows:

Many of these types are not used to any great extent in electric lighting, and it is therefore not necessary to consider them. The forms of boiler commonly employed in electric lighting are: The water-tube boiler, the internally fired direct firetube marine boiler, the locomotive type of boiler, and the plain horizontal tubular boiler. It has already been stated that watertube boilers of the Babcock & Wilcox and other types are very extensively used in electric lighting. They possess the advantages of being quick-steaming, not liable to disastrous explosions, and easily repaired and transported in sections. But the objections to them are that they are rather expensive, and do not have much capacity for water or steam, and cannot, therefore, stand

violent fluctuations of load, which, however, are not likely to occur in electric lighting.

The Babcock & Wilcox water-tube boiler, shown in Figs. 13, 14, and 15 is very generally used for electric lighting and other purposes in this country and many foreign countries. This and other similar types of boiler consist of a large number of parallel iron tubes joined at their ends by "headers," or connecting pieces of cast or wrought iron. The latter form is shown in Fig. 15, being required in high-pressure boilers. These tubes are ordinarily four inches in diameter, and are placed at a distance apart

about equal to their diameter. The tubes are "staggered," or arranged so that each tube is immediately over the space between two tubes in the row below. This has the effect of thoroughly abstracting the heat from the products of combustion. The mass of tubes are connected at both ends to the long horizontal steam and water drum above, the water-level being kept at such a height that this drum is about half full, as shown. At the rear the tubes are connected to the mud-drum below, into which the dirt, scale, etc., settles.

The path of the products of combustion is shown in Fig. 13.

They first pass directly upward from the grate, through all the water-tubes, being obliged to take this path by the bridge-wall at the back of the fire-box and a baffle-plate or partition which

surrounds the tubes, and forms an extension of the bridge-wall. About half-way between this wall and the rear end of the tubes is another baffle-plate, above which is a hanging wall of brick. These, together, cause the gases to pass downward through the tubes, and finally upward again at the back, thus flowing three times through the entire mass of tubes.

The circulation of the water is also very effective in these types of boiler. The inclined position of the tubes causes the heated water to flow from the rear toward the front of the boiler, thus traveling in a direction opposite to that of the gases. In this way the water is acted upon by hotter gases the higher its own temperature becomes.

There are many other well-known types of water-tube boilers. which are similar in principle but differ considerably in details of construction. Among these may be mentioned the Root, (Fig. 16 a) National, Heine, and Sterling.

In Europe the Steinmuller and other forms of water-tube boiler are used in addition to the Babcock & Wilcox, which

latter is as widely used there as in America. All these types of

water-tube boilers are employed in electric lighting; in fact, it is one of their most important applications.

Cylindrical or Horizontal-Tubular Boilers. A typical form is shown in Fig. 17, and consists of a cylindrical shell, closed at the ends by two flat tube-plates, through which the fire-tubes extend from one end to the other. The diameter of the fire-tubes is usually about 3 or 4 inches. Nearly two-thirds of the volume of the boiler is filled with water, the remaining space being reserved for the steam. The water-level is 6 to 8 inches above

the top row of tubes. The tubes act as stays for the tubeplates below the water-line; but above the water-level the flat plates must be stayed by through rods from one plate to the other, or by diagonal stays to the shell of the boiler.

The grate is under the front end of the boiler, and the products of combustion pass back under the boiler. A bridge-wall at the rear end of the grate is arranged to throw the gases into contact with the boiler. The gases return through the tubes, and pass out by the up-take, or flue leading to the chimney. The boiler is supported by cast-iron brackets, which are riveted to the shell, and rest on the side walls. A vertical steam-dome projects from the top of the boiler from which the steam is drawn. These boilers are made in sizes from about 3 feet in diameter and 7 feet long, having 12 horse-power capacity, to 7 feet in diameter and 20 feet long, having 200 horse-power capacity.