boilers, in that each featured three cylindrical water chambers arranged to form a triangle or, viewed from one end, an inverted "V" shape: the entire boiler was filled with water but for the upper part of the top chamber, which allowed for the collection of steam, and was connected by two banks of steam-generating tubes to the two lower chambers, between which was a furnace. Water tube boilers could operate at higher pressures and were much lighter than locomotive boilers, also known as "fire tube boilers" or, when used in ships, as "marine boilers". In these, water was contained in a single drum through which tubes carried exhaust gases from a furnace: a locomotive boiler had to be constructed from heavier gauge materials, since the greater size of the single drum required a thicker shell, and, while the tubes in a water tube boiler were subject only to tension from the steam and pressurised water within, a locomotive boiler's tubes were subject to compression from without, again requiring thicker materials. In the Normand boiler, the tubes were comparatively straight and a portion of those in the inner and outer rows of each bank were formed into "tube walls" to direct hot gases generated by the furnace through the boiler. In the Reed boiler, the tubes were bent into pronounced curves of varying radii to maximise surface area and therefore steam production, and baffles were used to direct hot gases. The lowest section of the lowest tubes of Reed boilers was originally bent into tight, "wavy" curves, also to maximise surface area, but this was discontinued by 1901 as it inhibited the flow of water and hence also steam. Further, the external diameter of the tubes tapered at their lower ends from inches (27 mm) to inch (22 mm) to improve the passage of hot gases between them. They were connected perpendicularly to the chambers at each end, as were the tubes in the Normand boiler, to reduce stress. However, in the Reed boiler these connections were made by hemispherical faces, which allowed "a certain angular play". The tubes were secured by nuts inside the chambers at each end. Handholes gave access to the bottom chambers, and a manhole gave access to the top chamber, allowing rapid replacement of defective tubes. In both types of boiler, the steam-generating tubes joined the top chamber below the designed water line to stop them