=Hot-water System.=--The principles of hot-water heating are very simple. Given a circuit of pipes filled with water, on heating the lower part of the circuit the water, becoming warmer, will rise, circulate, and heat the pipes in which it is contained, thus warming the air in contact with the pipes. The lower part of the circuit of pipe begins in the furnace or heater, and the other parts of the circuit are conducted through the various rooms and halls throughout the house to the uppermost story. The pipes need not be straight all through; hence, to secure a larger area for heating, they are convoluted within the furnace, and also in the rooms, where the convoluted pipes are called _radiators_. The water may be warmed by the low- or high-pressure system; in the latter, pipes of small diameter may be employed; while in the former, pipes of a large diameter will be required. The character, etc., of the boilers, furnace, pipes, etc., cannot be gone into here. =Steam-heating System.=--The principle of steam heating does not differ from that of the hot-water system. Here the pressure is greater and steam is employed instead of water. The steam gives a greater degree of heat, but the pipes must be stronger and able to withstand the pressure. There are also combinations of steam and hot-water heating. For large houses either steam or hot-water heating is the best means of warming, and, if properly constructed and cared for, quite healthy.[17] FOOTNOTES: [16] Great care should be taken that the air box is not placed in contaminated soil or where it may become filled with stagnant or polluted water.--EDITOR. [17] See Chapter XI for practical notes on cost of installation of these three conveyed systems--hot-air, hot-water, and steam.--EDITOR. CHAPTER IV =Disposal of Sewage= =Waste Products.=--There is a large amount of waste products in human and social economy. The products of combustion, such as ashes, cinders, etc.; the products of street sweepings and waste from houses, as dust, rubbish, paper, etc.; the waste from various trades; the waste from kitchens, e. g., scraps of food, etc.; the waste water from the cleansing processes of individuals, domestic animals, clothing, etc.; and, finally, the excreta--urine and faeces--of man and animals; all these are waste products that cannot be left undisposed of, more especially in cities, and wherever a large number of people congregate. All waste products are classified into three distinct groups: (1) refuse, (2) garbage, and (3) sewage.