Heating or Cooling of Buildings by means of Currents of Air. 139 



escape by couduction through the air passages. It is not my 

 present object to detei'mine how nearly in practice this degree 

 of oeconomy may be approximated to ; but to point out how the 

 limit which has hitherto appeared absolute may be surpassed, 

 and a current of warm air at such a temperature as is convenient 

 for heating and ventilating a building may be obtained mecha- 

 nically, either by water power without any consumption of coals, 

 or by means of a steam-engine di-iven by a fire burning actually 

 less coals than are capable of generating by their combustion the 

 required heat ; and secondly, to show how, with similar mecha- 

 nical means, currents of cold air, such as might undoubtedly be 

 used with great advantage to health and comfort for coohng 

 houses in tropical countries*, may be produced by motive power 

 requiring (if derived from heat by means of steam-engines) the 

 consumption of less coals perhaps than are used constantly for 

 warming houses in this country. 



In the mathematical investigation communicated with this 

 paper, it is shown in the first place, according to the general 

 principles of the dynamical theory of heat, that any substance 

 may be heated thirty degrees (Fahr.) above the atmospheric 

 temperature by means of a properly contrived machine, driven 

 by an agent spending not more than about 3 jth of energy of 

 the heat thus communicated ; and that a corresponding machine, 

 or the same machine worked backwards, may be employed to 

 produce cooling effects, requiring about the same expenditure of 

 energy in working it to cool the same substance through a 

 similar range of tempei'ature. When a body is heated by such 

 means, about f jths of the heat is drawn from surrounding 

 objects, and ^'jth is created by the action of the agent ; and 

 when a body is cooled by the corresponding process, the whole 

 heat abstracted from it, together with a quantity created by the 



* The mode of action and apparatus proposed for this purpose differs 

 from that proposed originally by Professor Piazzi Smyth for the same pur- 

 pose, only in the use of an egress cylinder, by which the air is made to do 

 work by its extra ])ressure and by expansion in passing from the reservoir 

 to the locality where it is wanted, which not only saves a great proportion 

 of the motive power that would be required were the aii- allowed simply to 

 escape through a passage, regulated by a stopcock or otherwise, but is ab- 

 solutely essential to the success of the project, as it has been demonstrated 

 by Mr. Joule and the author of this communication, that the cold of ex- 

 |>anKion would be so nearly compensated by tlie heat generated by friction, 

 when the air is allowed to rush out without doing work, as to give not a 

 tenth of a degree of cooling eti'ect in api)aratus planned for 30 degrees. 

 The use of an egress cylinder has (as the meeting was informed by Mr. 

 Macqufnn Rankine) recently been introduced into plans adopted by a com- 

 mittee of the Uritisli Association appointed to consider tlie ])racticability of 

 Professor Piazzi Smyth's suggestion, with a view to recommending it to 

 government for public buildiiuKs in India. 



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