140 Prof. Thomson on the (Economy of the 



agent, equal to about ^^i\\ of this amount, is given out to the 

 surrounding objects. 



A very good steam-engine converts about j'oth 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 tempera- 

 ture by the expenditure of only i jths, or yths, that is, less than 

 one-third of the coal that would be required to produce the same 

 elevation of temperature with perfect oeconomy in a direct pro- 

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

 means of the proposed machine the stated elevation of tempera- 

 ture, with the expenditure of ^^jth 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 be either like a steam-engine, founded on the 

 evaporation and recondensatiou of a liquid (perhaps some liquid 

 of which the boiling-point is lower than that of water), or an air- 

 engine of some kind. If the substance is to be heated or cooled by 

 air, it will be convenient to choose this itself as the medium 

 operated on in the machine. For carrying out the proposed 

 " object, including the discharge of the air into the locality where 

 it is wanted, the following general plan was given as likely to be 

 found practicable. Two cylinders, each provided with a piston, 

 ports, valves, and expansion gearing, like a high-pressure double- 

 acting steam- engine, are used ; one of them to pass air from the 

 atmosphere into a large receiver, and the other to remove air 

 from this receiver and discharge into the locality where it is 

 wanted. The first, or ingress cylinder, and the receiver should 

 be kept with their contents as nearly as possible at the atmo- 

 spheric temperature, and for this purpose ought to be of good 

 conducting material, as thin as is consistent with the requisite 

 strength, and formed so as to expose as much external surface as 

 possible to the atmosphere, or still better, to a stream of water. 

 The egress cylinder ought to be protected as much as possible 

 from thermal communication with the atmosphere or surround- 

 ing objects. According as the air is to be heated or cooled, the 

 pistons and valve gearing must be worked so as to keep the 

 pressm'c in the receiver below or above that of the atmosphere. 

 If the two cylinders be of equal dimensions, the arrangement 

 when the air is to be heated would be as follows : — The two pistons 

 working at the same rate, air is to be admitted freely from the 

 atmosphere into the ingress cylinder, until a certain fraction of 

 the stroke, depending on the heating effect required, is per- 

 formed, then the entrance port is to be shut, so that during the 

 remainder of the stroke the air may expand down to the pressure 



