UNDULATORY FORCES. HEAT. 



[HEATING BY STEAM. 



team of a density exceeding that of the atmosphere, 

 has been extensively used in what is called the expansive 

 engine. As the mechanical details of the steam-engine 

 will be fully entered into hereafter, we confine ourselves 

 to stating, that in engines of this class, the steam, after 

 being supplied to the cylinder of the engine, is allowed 

 to expand till the end of the stroke is arrived at ; and 

 thus its pressure is gradually diminished, whilst its bulk 

 is as gradually increased. In other forms of this class 

 of engine, two cylinders, of different sizes, are provided, 

 into the smaller of which, the high-pressure steam is 

 first introduced. As soon as the stroke of the piston is 

 completed in this cylinder, the steam is allowed to escape 

 into one of larger size ; and there exercising its expan- 

 sive power, a further mechanical effect is gained ; and 

 thus the same weight of steam is made to do double 

 duty whilst expanding to its normal bulk at 212 F. 

 This employment of steam is highly economical where 

 water can be obtained for condensing purposes. If very 

 high-pressure steam be employed, as in locomotives, it 

 can be similarly expanded, and is then allowed to escape 

 at a lower pressure in the usual manner, instead of 

 being condensed, as in the low-pressure engines. Great 

 economy in the consumption of fuel is thus effected ; and 

 the application of this plan has materially lessened the 

 cost of motive power on railways. In most marine 

 engines, steam of comparatively low pressure is em- 

 ployed, varying from ten to twenty pounds on the 

 square inch, above the atmospheric pressure ; and after 

 the steam has exerted its mechanical effect on one side 

 of the piston, it is allowed to escape into the condenser, 

 where, by coming in contact with cold water, it regains 

 a liquid state, by the abstraction of its latent heat. 



In an earlier part of this section, we have already 

 directed attention to the doctrine of latent heat ; we 

 shall, however, inquire more fully into that law as in- 

 volved in the production and condensation of steam. 



If water be kept boiling in an open vessel, a thermo- 

 meter immersed in the liquid will not indicate any rise 

 of temperature beyond the boiling point of 212F. If 

 another thermometer be immersed in the steam which 

 passes off, it will be found to register the same tem- 

 perature. It therefore follows, that the steam, after 

 attaining a temperature of 212, must carry off all the 

 caloric which passes into the liquid ; and this must pass 

 off in a latent state, because it cannot be appreciated, by 

 any instrument, in a sensible form. The attempt to 

 ascertain the amount of latent heat contained in steam, 

 has received the attention of some of the most able ex- 

 perimenters, who have arrived at various results. It is, 

 however, generally considered, that 1000 of heat, ac- 

 cording to the scale of Fahrenheit, are absorbed, and 

 become latent, during the formation of steam. As an 

 illustration of the mode of conducting such investiga- 

 tions, the following experiment is suggested : 



Experiment 32. Allow steam to pass through a pewter 

 worm contained in a vessel holding a known weight of 

 cold water say at a temperature of 60 F. As soon as 

 much condensed water, equal to an eighth part of the 

 bulk of the cold water, has been received from the worm 

 of the condenser, ascertain the temperature of the 

 water surrounding the -worm. It will have risen to 

 about 188. Now, as one part of water, in the form of 

 steam, has heated eight parts of water from a tempera- 

 ture of 60 to 188, or a difference of 128, it follows 

 that the latent heat of steam is not less than eight times 

 128, or 1024 ; but as there is a difference between the 

 sensible heat of the water surrounding the condenser, 

 and that of the steam wliich has passed through it, of 

 212 188, which is equal to 24, this must be deducted ; 

 and hence a carefully conducted experiment gives 1000 

 as the latent heat of steam at 212 F. 



In the condensing steam-engine, cold water is em- 

 ployed for the purpose of absorbing, or taking away the 

 latent heat of the waste steam. As, however, it is 

 necessary that the temperature added to the water 

 should be as little as possible, so as to keep the vacuum 

 perfect, a much larger proportion of water must be em- 

 ployed than we named in Experiment 32, because eight 



parts were there heated to a temperature of 188. As, 

 therefore, the temperature of the condenser is never 

 allowed to exceed 110" if possible, about twenty times 

 as much water is employed as the weight of steam which 

 has to be condensed. 



It will not be required that we should hero enter into 

 the question of economy in the employment of super- 

 heated steam, because we shall have to investigate that 

 subject when treating on the steam-engine. 



We shall next proceed to speak of the use of steam for 

 heating and warming purposes ; and in this we liavo an 

 interesting instance of the application of the laws of 

 latent heat. 



By Experiment 32, we found that steam at 212 will 

 raise the temperature of eight times its weight of water, 

 at ordinary temperatures, by 128. And here we have a 

 most valuable means of conveying heat to any point we 

 wish, without the necessity of erecting numerous fur 

 naces, or other heating apparatus. Indeed, if stcam- 

 pipes are carefully covered with some non-conducting 

 material, such as felt or wood, the steam may be con- 

 veyed to very great distances without undergoing con- 

 densation. As an instance of this, the whole of the 

 steam-engines at the Exhibition in Hyde Park, in 1851, 

 were worked by steam thus conveyed from boilers in a 

 distant shed ; and the same plan is now adopted at the 

 Crystal Palace, at Sydenham. In most of the large 

 bleach and printing-works in the north of England and 

 in Scotland, the whole of the liquids employed are heated 

 by means of steam conveyed into them through pipes, 

 or by steam being admitted into an external vessel, 

 surrounding that which has to be raised in temperature. 

 In many cases, the waste steam passing from a high- 

 pressure engine, is made to raise the temperature of the 

 water supplied to the boiler; and this plan greatly 

 economises the consumption of fuel. 



In our remarks on warming and ventilation, we 

 deferred describing the use of steam, from the idea that 

 such would be most conveniently dealt with under this 

 head. For such purposes, an ordinary steam boiler is 

 used, from which iron pipes are carried. These are 

 carefully coated with a non-conductor, until they arrive 

 at the place where the heat of the steam is required. 

 By the radiation of the heat of the steam, at a tem- 

 perature of 200, from the iron pipe, a room sixteen 

 feet square by twelve feet high, and of ordinary con- 

 struction, can be well warmed and ventilated by means 

 of a steam-pipe, twenty feet long and four inches in 

 diameter, or by any other form of metal casing exposing 

 an external surface of twenty square feet. According to 

 this calculation, which was made by Dr. Arnott, it 

 follows, that in rooms or large buildings having the usual 

 amount of window and wall surface, every square foot 

 of steam piping, at the temperature of about 200, will 

 warm, to an agreeable extent, a space of 150 cubic feet, 

 allowing for imperfect fittings, and for the admission, for 

 the purpose of ventilation, of air, which is also heated 

 during its passage into the apartment. This estimate 

 supposes the external air to be at a temperature below 

 freezing point. Of course, when the external temperature 

 Is higher, the same heating surface will have a greater 

 warming effect on the air in the apartment. As we have 

 already explained, the amount of window surface mate- 

 rially affects the success of any heating arrangement. 



We have hinted at various objections which apply to 

 the use of steam in churches, theatres, <kc. : these, how- 

 ever, do not exist in manufactories where engineers are 

 always employed. Hence, steam is much used in such 

 places as a drying agent, and for various objects which 

 our limited space will not permit us to detail 



TABLES OF THE EFFECTS OF HEAT ON 

 BODIES. 



HAVING endeavoured to lay before the reader an exposi- 

 tion of the laws of heat, and their general application, we 

 conclude our section on this subject by supplying 

 various tables, skeletons of which have already appeared 

 in some of the previous pages. 



