That heat escapes rapidly by radiation from the outside of steam boilers containing steam of high pressure, when the surface is unprotected, is familiar to all. That clothing or covering with wood, or felt, or with a cement of asbestos, gypsum, or some other feeble conductor of heat~ lowers the temperature exposed to the air, and so diminishes the loss by radiation, is also well known.
What is not so generally known, or so accessible, if the knowledge exist at all~ is the exact ratio of loss under certain definite conditions, such as we often meet in practice, and the economic value of certain admissible kinds of clothing. The saving effected by clothing boilers, pipes and other vessels containing steam, cannot be considered as inversely proportioned to the conducting power of the material used for clothing. The outer surface is more or less enlarged, and radiation, if less active, goes out from the enlarged surface. If the material of the outer covering be a better radiator than iron, as it will be of necessity if • poorer conductor, the loss may be very great while the radiating surface feels only moderately warm to the hand. Just what the loss is in a given boiler under familiar conditions, what ratio this loss bears to the steam generating power of the boiler, and what saving can be effected by clothing of some convenient kind, are facts not generally known, and such data as have been established by observation and experiment, are neither very accessible nor easily applied in practice.
Nearly all boilers of much importance in use for stationary purposes, are effectively protected by brickwork, with ashes or other similar material on top. Locomotives, exposed as they are to all the rigors of the elements, to wind, snow and rain, have usually something more than half of their exterior in some degree protected with a covering, generally of wood, cased with sheet iron.