From the PREFACE. The dearth of practical information on steam heating, and the want felt by the young steam-fitter, in almost all branches of his trade, has suggested to me the necessity of explaining, so far as lies in my power, some of the many questions which often arise. This volume has no scientific pretensions beyond what are actually necessary to explain a few laws, which affect the action of steam, water, and air, within pipes; and is simply a "Vade Mecum" of practical results to the fitter which the trade has tacitly adopted, and from repeated failures at first it has come to practical success eventually. These results I call "Hints," since I make many assertions I do not explain, which are known to be facts, and which will be of more real value to a beginner, than a long-drawn exhortation of both sides of the question, defeating its own object by leaving the student undecided. From the INTRODUCTION. Within twenty years, the warming of buildings with steam carried through pipes became a science; previously, it was a chaotic mass of pipes, and principles. A low-pressure gravity apparatus is the most healthful, economical, and perfect heating appliance known, and may be constructed to heat a single room, or the largest building, with a uniformity which cannot be attained by any other means. By a gravity apparatus is meant, one without an outlet, whose circulation is perfect, wasting no water, and requiring no "mechanical means" to return the water to the "boiler." It may be likened to the circulation of the blood-the boiler being the heart; the "steam-pipes," the arteries; and the return-pipes, the veins: thus carrying heat and life into every part of a building. When reference is made to steam-pressure in this volume, it is understood to mean "pressure above the atmosphere." Nearly all tables of reference on steam are given in "absolute" pressures-namely, pressures including the pressure of the atmosphere-which unapparent pressure has to be overcome before it is appreciable on a steam-gauge; and, as the steam-fitter has little, if anything, to do with pressures below atmosphere, the tables, etc., herein used will be modified, to commence at atmospheric pressure (14 7/10 pounds of the absolute scale), thus conveying comparison in the "ordinary terms" to which the steam-fitter is accustomed; and preventing the necessity of a mental calculation, which always involves fractions, and enjoins a task which should not be thrown on a beginner. Therefore, all pressures mentioned will be "apparent pressures"-namely, pressures that would be indicated by a "properly regulated steam-gauge."