HOT- WATER APPARATUS.] 



UNDULATORY FORCES. HEAT. 



35 



Hot water, conveyed in iron pipes, is generally 

 adopted, at the present day, as a source of artificial heat 

 in most of our public buildings ; and many points of 

 considerable scientific interest are involved in the con- 

 struction and working of the arrangement. In Mr. 

 Perkins' apparatus, tubes, of an inch in diameter, are 

 formed into a coil, varying in size according to the ex- 

 tent of cubic dimension to be heated. The pipe, rising 

 from the surface of the coil, is extended generally over 

 the flooring of the building, and returns to the coil at its 

 lower part. The coil itself is fixed in a furnace of any 

 kind. The water in the coil, on becoming heated, rises 

 to the upper tube, and traverses the pipes throughout 

 the edifice, so long as its temperature is higher than that 

 of the surrounding air. Having been cooled, the expan- 

 sion which it had, when heated, is lost, and becoming 

 denser, it returns to the coil, again to pursue the same 

 course on receiving a fresh accession of heat. The 

 philosophy of the process is easily understood, when we 

 bear in mind, that the hot water being light, naturally 

 ascends ; on cooling, however, it contracts, and descends 

 to the coil. 



The bore of the pipe in Mr. Perkins' arrangement 

 rarely exceeds half an inch. Numerous modifications 

 of this plan have been lately adopted. Cast-iron pipes, 

 of three or four inches in the bore, and peculiarly shaped 

 boilers, are substituted for the small-bore pipes and coil, 

 at the present time ; and the use of these arrangement* 

 is generally spoken of as effective, and requiring but 

 little attention. By the employment of pipes of large 

 dimen- : >ns, such as those just described, the circulation 

 of the hot water is rapidly effected, in a manner which 

 could not be obtained in the smaller kind. Indeed, we 

 have frequently seen the small-pipe coils red-hot, and 

 luive observed ft temperature of 4UO F., at a distance of 

 fifty feet from t)ie furnace, in the pipe conveying the 

 upward current of water. In the boiler and large-pipe 

 arrangement, the temperature rarely exceeds, if it 

 reaches, 312 F. 



There is ft great advantage in the employment of hot 

 water over that of steam, for heating public buildings 

 in the freedom from risk of explosion. If a hot-water 

 apparatus should actually burst, not the least danger 

 could possibly arise ; the water, in fact, would merely now 

 out : thus the apparatus may at all times be left to itself, 

 and only requires that the furnace should be supplied 

 with fuel. In the use of steam (to which we shall refer 

 immediately), great care is required that a proper supply 

 of water should be kept up ; that the safety-valves should 

 be in proper condition ; and other details requiring intel- 

 ligence and care are involved. The expansion of the 

 water in the hot-water apparatus is always provided for, 

 by inserting in some part of the piping an upright tube, 

 through which the water is first introduced, on erecting 

 the apparatus. A space containing air is left inside this, 

 in 'which the water gradually rises as it is heated, and 

 expands. 



It would be beyond the plan of our work, were we to 

 enter into those practical details involved in applying 

 this apparatus, so far as questions of the size, ic. , re- 

 quired for special cases is concerned. This is a matter 

 which must be left to the engineer, and is one which 

 depends on various local and incidental circumstances, 

 which can only be determined as they arise. Our object 

 has been to present the philosophy of warming and ven- 

 tilation in such a manner, that the principles may be 

 understood and carried into effect by any workman of 

 ordinary intelligence, on the exercise of whoso skill and 

 judgment success must of course depend. 



STEAM, AND ITS APPLICATIONS. 



THB process of converting water into steam is so fami- 

 liar to all, as to require no description. The physical 

 changes which the liquid undergoes are, however, of the 

 highest importance, so far as the laws of it* change of 

 state are concerned. 



Evaporation takes place at nearly all observed tempe- 

 ratures. If a piece of ice be placed in the receiver of an 



air-pump, and a watch-glass, filled with strong sulphuric 

 acid, be placed near it, the ice will gradually evaporate, 

 provided that a good vacuum is maintained. Vapour pro- 

 duced at even low temperatures, exerts a pressure on a 

 column of mercury contained in a barometer tube ; and 

 as an illustration of this, the following experiment may 

 be tried : 



Experiment 31. Pour a little ether into a tube of 

 glass, sealed at one end, and about thirty-two inches long. 

 When the tube is completely moistened by the liquid, fill it 

 with mercury. If no vapour existed in the tube, the mercury 

 would stand as high as in the barometer, if the glass tube 

 were inverted into a cistern of mercury, and would act as a 

 barometer at the moment this experiment is tried. But 

 owing to the ether being converted into vapour, and so 

 partially occupying the upper portion of the glass, the 

 mercury will not stand at a height of tliirty inches, or 

 thereabouts, but be depressed nearly ten inches below 

 that point, owing to the elasticity of the vapour of the 

 ether. If water be employed, the depression of the 

 mercury will be very slight at low temperatures perhaps 

 not exceeding half an inch ; because it is less volatile 

 than ether. It will be thus perceived, that liquids not 

 only continually evaporate at low temperatures, but that 

 their vapour exerts a pressure on the barometric column 

 proportional to their ready evaporation, or easy conver- 

 sion into a vaporous state. 



It has been already explained, that the atmosphere 

 presses with a force of about fifteen pounds on every 

 square inch of surface at the level of the sea, in ordinary 

 circumstances. The vapour of water presses with an 

 equal force when of a temperature of 212, the barometer 

 indicating a height of thirty inches of mercury. The 

 annexed table give* the pressure which aqueous vapour 

 exerts at different temperatures, calculated in parts of 

 an atmosphere of fifteen pounds ; the column of mercury 

 which the vapour would sustain ; and the pressure rela- 

 tive to that of the atmosphere. 



Trmnrt,]r Prewure In Inchei of Mercury Pi-mure In Foundl 

 Atmmphfrci. lupported. per Square Inch. 



60 F. 0.01T 0.5 0.25 



120 0.130 3.6 1.75 



180 0.500 15.0 7.50 



312 1.000 30.0 15.00 



250 2.000 60.0 30.00 



275 3.000 90.0 45.00 



200 4.000 120.0 60.00 



305 6.000 150.0 75.00 



320 6.000 180.0 90.00 



It will be thus observed, that the pressure of steam 



increases in proportion as the temperature is raised ; but 



when the temperature exceeds that of the boiling point 



of water, the tension of the vapour rises more rapidly, 



for the same number of degrees, than between lower 



temperatures. 



It is generally reckoned that one cubic inch of water, 

 at ordinary temperatures, is expanded into 1,690 cubic 

 inches, on being converted into steam at a temperature 

 of 212 Fahrenheit ; and so long as that temperature is 

 maintained, the steam exerts a mechanical effect equal 

 to fifteen pounds pressure on every square inch of any 

 surface with which it is in contact. 



If, however, steam be confined in a vessel such, for 

 instance, as an ordinary steam boiler and the production 

 of vapour is carried on continuously, high-pressure steam 

 is produced, which exerts a greater pressure than that 

 afforded in an open vessel. Its density is thus increased, 

 the steam being compressed into a small bulk, in pro- 

 portion as the pressure augments; and when the 

 steam thus formed exerts a force exceeding that of the 

 atmosphere by fifteen pounds, the vapour has been com- 

 pressed by the pressure into one-half of the space it 

 would occupy if allowed to escape freely into the open 

 air. The bulk of the vapour is thus inversely as its 

 density. In practice, 100 to 150 pounds per square inch 

 is the maximum pressure used in steam boilers ; and 

 these pressure* are chiefly confined to the locomotive 

 engines employed on railways. 

 Beside* being employed in the high-pressure engine, 



