170 Mr. Liddell's Concluding Report of the Exhibition. 



Some illustrations, afforded by the Air Engine, of general physical 

 principles, were also noticed. If the Air Engine be turned forwards, by 

 the application of power, and if no heat be applied, the space below the 

 plunger will become colder than the surrounding atmosphere, and the space 

 above hotter. Expenditure of work will be necessary to turn the engine, 

 after this difference of temperatures, contrary to that which is necessary 

 to cause the engine to turn forwards, has been established. If, however, 

 we prevent the temperature in one part from rising, and in the other from 

 sinking, the engine may be turned without the expenditure of any work, 

 (except what is necessary in an actual machine for overcoming friction, &c.) 

 One obvious way of retaining the two parts at the same temperature, 

 is to keep the machine immersed in a stream of water; but there is 

 another way in which this may be done, if we can find a solid body which 

 melts at the temperature at which it is required to retain the Engine. 

 For instance, let this temperature be 32° ; let a stream of water at 32° be 

 made to run across the upper part of the Engine, and let the lower part 

 of the vessel containing the plunger, which is protected from the stream, 

 be held in a bason of water at 32°. When the Engine is turned forwards, 

 heat will be taken from the space below the plunger and deposited in the 

 space above. Now, this heat must be supplied by the water in the bason, 

 which will, therefore, be gradually converted into ice at 32°. Hence we 

 see that water at 32° may be converted into ice at 32°, without the 

 expenditure of any work. This may also be very easily proved in the 

 following manner: — 



Let a syringe be constructed of perfectly non-conducting materials, 

 except the lower end of the cylinder, which is to be stopped by a solid 

 plate, a perfect conductor. The syringe being at first full of air, at 

 atmospheric pressure, and at the temperature of 32° ; let the lower end be 

 dipped in a stream of water at 32°, and the piston be pushed down. Let 

 the syringe be then placed with its lower end in a bason of water at 32°, 

 and the piston be allowed to rise. The mechanical effect given out in 

 this part of the operation will be equal to the work spent in the former, 

 and a portion of the water in the bason will be turned into ice. 



Note. — To avoid perplexity, in the account which was given, it was 

 supposed that the temperature of the air is always the same as that of the 

 vessel in which it is contained, which will only be strictly true, even were 

 the action of the plunger perfect in altering the temperature of the air, 

 when the motion is very slow. 



April 28th, 1847. — The Society met for the last time this Session. — 

 The Vice-President in the Chair. 



Mr. Liddell made his concluding report on the winding up of the affairs 

 of the Society's exhibition at the beginning of the year. All the accounts 



