_Classes of Boilers._--Boilers generally may be divided into two distinct classes, one comprising those which are generally called "tank" boilers, containing relatively large quantities of water, and the other those which are generally called "water-tube" boilers, in which the water is mainly contained in numerous comparatively small tubes. There are, however, some types of boiler which combine to some extent the properties of both these classes. Each class has its representatives amongst both land and marine boilers. In "tank" boilers the outer shell is wholly or partially cylindrical, this form being one in which the necessary strength can be obtained without the use of a large number of stays. The boilers are generally internally fired, the furnace plates being surrounded with water and forming the most efficient portion of the heating surfaces. On leaving the furnace the products of combustion are led into a chamber and thence through flues or through numerous small tubes which serve to transmit some of the heat of combustion to the water contained in the boiler. In "water-tube" boilers the fire is usually placed under a collection of tubes containing water and forming the major portion of the heating surface of the boiler. Both the fire and the tubes are enclosed in an outer casing of brickwork or other fire-resisting substance. In some forms of water-tube boiler the fire is entirely surrounded by water-tubes and the casing is in no part exposed to the direct action of the fire. In "tank" boilers generally no difficulty is experienced in keeping all the heating surfaces in close contact with water, but in "water-tube" boilers special provision has to be made in the design for maintaining the circulation of water through the tubes. (For "flash" boilers see MOTOR VEHICLES, and for domestic hot-water boilers HEATING.) Lancashire.