ANNUAL ADDRESS. VII. 



small machine in which a concealed magnetic needle was rotated 

 by means of clock work under a thin hollow stand. This magnet 

 caused the actual revolution of a light conical pendulum, the 

 motion of which was ostensibly produced by a circle of perma- 

 nent magnets in sight of the on-looker. Such things are 

 interesting as toys, and dangerous as frauds ; but they have no 

 scientific importance. In the second class before referred to, 

 and to which Tripler's case (if it were not otherwise impossible) 

 belongs, the so-called perpetual motion is produced either by the 

 direct utilisation of the heat of the sun's rays, by storing up 

 heat during the day time, as in Ericson's Solar Engine, to be 

 given out when the temperature is lower at night ; or by some 

 other system of utilising natural heat energy, just as it is 

 employed in any other heat engine. Instead, however, of the 

 temperature range being limited at the cold end by about 100 

 degrees Fah., as in the steam engine, it may possibly be limited 

 by the lowest temperature attained by the atmosphere. 



If, however, the effective work of the heat engine can be 

 increased, and its lower range be economically extended to 

 condensing temperatures produced artificially (as claims which 

 have been made on behalf of the use of liquid air would lead the 

 general public to believe), then the application of the laws of 

 thermo-dynamics must be modified. 



It would be a most fascinating subject for contemplation if 

 we could assume that, through the instrumentality of liquid air 

 instead of water as the medium for our motive power, we could 

 construct a heat engine in which the range of temperature would 

 be altogether below the ordinary temperature of the atmosphere, 

 on the grounds that it is a liquid which boils at atmospheric 

 pressure 312 degrees below Fahrenheit's zero, and as a vapour or 

 gas occupies 800 times its liquid volume. Having a supply of 

 liquid air, it is certainly quite possible to drive an engine with 

 high-pressure air as a vapour or gas, and for the heat necessary 

 to produce such evaporation to be abstracted from its surround- 

 ings, finally exhausting the expanded medium so cold as to be 

 still below the freezing point of water. Numbers of estimates 



