Pbofessor Thomson on the Heating and Cooling of Buildings. 271 



be either like a steam engine, founded on the evaporation and re-conden- 

 sation 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 sub- 

 stance to be heated or cooled be 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 atmospheric 

 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 

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

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

 {ha receiver below, or above, that of the atmosphere. If the 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 performed, then the entrance port is to be shut, so that during the 

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

 receiver, into which, by the opening of another valve, it is to be admitted 

 in the reverse stroke ; while the egress cylinder * is to draw air freely 

 from the receiver through the whole of each stroke on one side or tho 

 other of its piston, and in the reverse strokes first to compress this air 

 to the atmospheric pressure (and so beat it as required), and then dis- 

 charge it into a pipe leading to the locality where it is to be used. If 

 it be required to heat the air from 50" to 80" Fahr., the ratio of expan- 

 sion to the whole stroke in the egress cylinder would be y'ji'j, the pressure 

 of the air in the receiver would be •j8^'|, of that of the atmosphere (about 

 2-7 lbs. on the square inch below the atmospheric pressure), and the 



* In this case tlie egress cylinder acts merely as an air pump, to draw air from 

 the receiver and discharge it into the locality where it is wanted, and the valves 

 required for this purpose might be ordinary self-acting pump-valves. A similar 

 remark applies to the action of the ingress cylinder in the use of the apparatus for 

 producing a cooling effect on the air transmitted, which will then be that of a 

 compressing air-pump to force air from the atmos])hcre into the receiver. But 

 in order that the same a])paratus may be used for the double purpose of heating or 

 cooling, as may lie required at dili'erent seasons, it will be convenient to have tho 

 valves of each cylinder worked mechanically, like those of a steam engine. 



