English: Longitudinal- and cross-section diagrams of a Reed water tube boiler as patented in 1893 by Joseph William Reed, manager of the engine works at Palmers Shipbuilding and Iron Company of Jarrow, England, and manufactured by them; in this instance as fitted to HMS Lightning, also built by Palmers and launched in 1895.

"We illustrate the boilers of the Lightning, which are of the three chamber class, with four large external downcast pipes. Probably Mr. Reed will find that these are superfluous after some further experience. Fig. 1 is a longitudinal section; Fig. 2 a cross section; ... In the figures, 8 are the fire doors, 9 the cleaning doors, 10 the ashpan doors, and 11 the circulating pipes connecting the lower chambers A with the upper chambers A¹.

The downcomers were arranged primarily to give a certain supply of water to the bottom ends of the tubes, but at the same time they form a substantial part of the framework of the boiler. The tubes are of solid drawn steel galvanised externally by electro-deposition. The lower chamber is of cylindrical cross section with a flattened lower side, and provided with handholes for access; a manhole is provided in the upper chamber. The tubes are readily secured, and in the event of damage can be removed and replaced in a very short time; at the same time a very secure joint is made, for in the vessels referred to there are in all upwards of 30,000 joints, and though they have been many times under steam, not a single leak has occurred. Baffle plates B are provided, as shown, to assist in directing the flow of the gases. The furnace and tube casings are double, with an air space between, the fire side being lined in an effective manner with an asbestos composition, the outer casing being quite cool; even when under heavy forced draught, the hand can be borne comfortably on the boiler front. The lower part, near the grate, is lined with fire quarls. The furnace has three doors arranged to close and fasten automatically. The arrangements are such that all air supply to the furnace passes through automatic doors, all precautions being taken to prevent flame being forced into the stokehold, in the event of an accident to a tube. These precautions were considered desirable at the time of designing the work, owing to the unfortunate accidents that had then occurred in some other parts of the country. In this case the necessity for such precautions has never arisen, for although the twelve boilers in these three vessels, the Janus, Lightning, and Porcupine, have been many times under steam under forced draught, some up to as far as 5½" [140 mm] of water, there has not been a single accident or leak of any kind.

There are four boilers in each vessel, each having its separate main feed pump, the feeding being controlled by Reed's automatic feed regulator. In the upper chamber and dome a simple but effective form of separator is fitted, which ensured freedom from priming even at the highest rates of evaporation.

During the experiments on shore with the first boiler of these vessels, an evaporation at the rate of 28,300 lb. [12.84 tonnes] of water per hour was obtained. The boilers are capable of supplying more steam than the engines can take ..." (Anon., "Reed's water-tube boilers", The Engineer, 14 February 1896, p. 172)

The number of holes in the top chamber illustrated in the longitudinal section diagram indicates that the boiler contained a total of 694 steam-generating tubes.