HEATING BY HOT- WATER PIPES. 



191 



same, in various parts of the houses. In 

 some cases we have used an evaporating- 

 pan placed under the return-pipe, so that it 

 may be half covered with liquid, enriched 

 with both pigeon's dung and guano. — 

 Vide section 4, " Heating by Tanks and 

 Gutters." From this will be seen the 

 advantages of combining the tank or 

 gutter principle with that of hot-water 

 pipes as hitherto applied. 



Heating by hot water, for horticultural 

 purposes, has many advantages over 

 steam, because by it a sufficient and more 

 uniform temperature can be maintained 

 at less expense, and with no danger. It 

 is seldom the case, nor is it at all neces- 

 sary, that hot-water pipes should reach 

 212°; for if they did so, steam would be 

 generated, and would escape at the top of 

 an open boiler, or at the vent air-hole or 

 expansion-tube in one that is closed. To 

 obtain steam in the first instance, the 

 water in the boiler must reach 212° at 

 the least ; and to keep it at or above 

 that point, must of necessity require a 

 greater consumption of fuel. On this 

 subject, Hood's remarks are excellent. 

 " The weight of steam at the temperature 

 of 212°, compared with the weight of 

 water at 212°, is about as 1 to 1694 ; so 

 that a pipe which is filled with water at 

 212°, contains 1694 times as much matter 

 as one of equal size filled with steam. If 

 the source of heat be withdrawn from the 

 steam-pipes, the temperature will soon 

 fall below 212° and the steam immedi- 

 ately in contact with the pipes will con- 

 dense ; but, in condensing, the steam 

 parts with its latent heat, — and this heat, 

 in passing from the latent to the sensible 

 state, will again raise the temperature of 

 the pipes. But as soon as they are a 

 second time cooled down below 212°, a 

 farther portion of steam will condense, 

 and a farther quantity of latent heat will 

 pass into the state of heat of tempera- 

 ture," — that is, a state of heat measurable 

 by a thermometer, whereas latent heat 

 is incapable of being measured by any 

 instrument whatever, — " and so on, 

 until the whole quantity of latent heat 

 has been abstracted, and the whole of 

 the steam condensed, in which state it 

 will possess just as much heating power 

 as a similar bulk of water at the like 

 temperature — that is, the same as a 

 quantity of water occupying 1-1 694th 



part of the space which the steam ori- 

 ginally did." 



By experiments made by the above 

 authority, it has been proved that a given 

 bulk of steam will lose as much of its 

 heat in one minute, as the same bulk of 

 water would in three hours and three 

 quarters. And farther, admitting that 

 the heat of cast-iron is nearly the same as 

 that of water, if two pipes of the same 

 calibre and thickness be filled, the one 

 with water and the other with steam, 

 each at 212° of temperature, the former 

 will contain 4.68 times as much heat as 

 the latter; therefore, if the steam-pipe 

 cools down to 60° in one hour, the water- 

 pipe will take four hours and a half to 

 cool down to the same point. In a hot- 

 water apparatus, we have, in addition to 

 the above, the heat from the water in the 

 boiler, and of the heated material in and 

 about the furnace, -which continues to 

 give out heat for a long time after the 

 fire is totally extinguished ; whereas in a 

 steam apparatus, under the same circum- 

 stances, we have no source of heat ex- 

 cepting the pipes by which it is con- 

 veyed — giving an advantage in favour 

 of hot water over steam as regards its 

 power of heating hothouses, and main- 

 taining heat, after the fire ceases to burn, 

 in nearly the proportion of 1 to 7 — that 

 is, hot water will circulate from six to 

 eight times longer than steam under the 

 above circumstances. 



Heat is given off from bodies by two 

 distinct processes — Radiation and Con- 

 duction. " In the radiation, the rays of 

 heat diverge in straight lines from every 

 part of a heated surface, and also from 

 extremely minute depths below such sur- 

 face. These rays, like rays of light, are 

 subject to the laws of refraction and re- 

 flection, and their intensity decreases as 

 the square of the distance. When we 

 approach an open fire, or the surface of a 

 stove, we feel its heat by radiation ; and 

 it has been ascertained that, at the ordi- 

 nary temperature of hot-water pipes, 

 about one-fourth of the total cooling 

 effect is due to radiation. But the amount 

 of radiation of a body heated above the 

 temperature of the surrounding atmos- 

 phere, depends greatly upon the nature 

 of its surface. If a vessel of hot water, 

 coated with lamp-black, radiate 100 parts 

 of heat within a given time, a similar 



