270 Professor Thomson on the Heating and Cooling of Buildings. 



show how, with similar mechanical means, currents of cold air, such as 

 might undoubtedly be used with great advantage to health and comfort 

 for cooling 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 above the atmospheric temperature by means of a properly con- 

 trived machine, driven by an agent spending not more than about ^^ 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 

 temperature. When a body is heated by such means, about §J of the 

 heat is drawn from surrounding objects, and Jj is created by the action 

 of the agent ; and when a body is cooled by the corresponding pro- 

 cess, the whole heat abstracted from it, together with a quantity created 

 by the agent, equal to about J^ of this amount, is given out to the sur- 

 rounding objects. 



A very good steam engine converts about J^ of the heat generated in 

 its furnace into mechanical effect ; and consequently, if employed to work 

 a machine of the kind described, might raise a substance thirty degrees 

 above the atmospheric temperature by the expenditure of only i^, or 

 f, that is, less than one-third of the coal that would be required to 

 produce the same elevation of temperature with perfect economy in a 

 direct process. If a water-wheel were employed, it would produce by 

 means of the proposed machine the stated elevation of temperature, with 

 the expenditure of -J^ of the work, which it would have to spend to 

 produce the same heating effect by friction. 



The machine by which such effects are to be produced must have the 

 properties of a " perfect thermo-dynamic engine," and in practice would 



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

 proposed originally by Professor Piazzi Smyth for the same purpose, only in the 

 use of an egress cylinder, by which the air is made to do work by its extra pressure 

 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 air allowed simply to escape through a passage, I'egulated by a stop-cock 

 or otherwise, but is absolutely 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 

 expansion would be so nearly compensated by the heat generated by friction, when 

 the air is allowed to rush out without doing work, as to give not two-tenths of a 

 degree of cooling efl'ect in apparatus planned for 30 degrees. The use of an egress 

 cylinder has (as the meeting was informed by Mr. Macquorn Rankine), recently 

 been introduced into plans adopted by a committee of the British Association 

 appointed to consider the practicability of Professor Piazzi Smyth's suggestion, 

 with a view to recommending it to government for public buildings in India. 



