(_d_) It may be premised that boilers have been made of various figures, each having its own peculiar advantages and defects. That which possesses the greatest degree of strength, is one of the shape of a cylinder. This form was originally introduced by Oliver Evans, in the United States. It will have its entire superiority when the fire is made beneath it in a furnace of masonry. But this method is not applicable to steam boats or locomotive engines. In these instances the weight of the separate furnace is an objection; when, therefore, this form is applied in them, the fire is usually made in a chamber of cylindric shape, within the boiler, and the smoke is conveyed to the chimney by flues, which pass through the water. These flues have, in some cases, been reduced to the size of small tubes, and of this method an example will be found in a subsequent part of this work.--A. E. (66.) Different methods have been, from time to time, suggested for indicating the level of the water in the boiler. We have already mentioned the two gauge-pipes used in the earlier steam engines (31.); and which are still generally continued. There are, however, some other methods which merit our attention. A weight, F (fig. 32.), half immersed in the water in the boiler, is supported by a wire, which, passing steam-tight through a small hole in the top, is connected by a flexible string or chain passing over a wheel, W, with a counterpoise, A, which is just sufficient to balance F, when half immersed. If F be raised above the water, A being lighter, will no longer balance it, and F will descend, pulling up A, and turning the wheel W. If, on the other hand, F be plunged deeper in the water, A, will more than balance it, and will pull it up. So that the only position in which F and A will balance each other is when F is half immersed. The wheel W is so adjusted, that when two pins, placed on its rim, are in the horizontal position, as in fig. 32., the water is at its proper level. Consequently it follows, that if the water rise above this level, the weight F is lifted, and A falls, so that the pins P P´ come into the position in fig. 33. If, on the other hand, the level of the water falls, F falls and A rises, so that the pins P P´ assume the position in fig. 34. Thus, in general, the position of the pins P P´ becomes an indication of the quantity of water in the boiler.