MECHANICS AND USEFUL ARTS. 43 



escape after use. The advantage of the one over the other re- 

 sults from the lower temperature required in the case of ammonia 

 to produce a given pressure, or from the higher pressure obtain- 

 able at a given temperature. These circumstances are favorable 

 to the economical action of the machine in two ways. In the tirst 

 place they considerably diminish the great waste of heat which 

 always takes place in the furnace of every engine driven by heat; 

 the waste, that is, which occurs through the chimney without 

 contributing in any manner to the operation of the machine. 

 This waste will be necessarily greater in proportion as the tire is 

 more strongly urged ; and it will be necessary to urge the lire in 

 proportion as the temperature is higher at which the boiler, or 

 vessel containing the elastic medium which furnishes the power, 

 has to be maintained. In the second phice, that great loss of 

 power to which the steam engine is subject, in consequence of the 

 high temperature at which the steam is discharged into the air, or 

 iiiio a condenser, is very materi|lly diminished in the engine 

 driven by ammoniacal gas. 



For instance, steam formed at the temperature of 150° C. 

 (302° F.) has a pressure of nearly 5 atmospheres (4.8). If 

 worked expansively, its pressure will fall to one atmosphere, 

 and its temperature to 100° C. (212° F.), after an increase of vol- 

 ume as one to 4. If, now, it is discharged into a condenser, 

 there is an abrupt fall of temperature of 50°, 60°, or 70°, without 

 any corresponding advantage. If it is discharged into the air, 

 this heat is just as much thrown away. In point of fact, when 

 steam of 5 atmospheres is discharged into the air at the pressure 

 of one, considerably more than half the power which it is theo- 

 retically capable of exerting is lost ; and when, at the same pres- 

 sure, it is discharged into a condenser, more than one quarter of 

 the power is in like manner thrown away. And as the expansion 

 given to steam is usually less than is here supposed, the loss 

 habitually suffered is materially greater. 



The ammoniacal solution affords a pressure of 5 atmospheres 

 at 80° C. (176° F.), and in dilating to 4 times its bulk, if it were a 

 perfectly dry gas, its temperature would fall below 0° C. But as 

 some vapor of water necessarily accompanies it, this is condensed 

 as the temperature falls, and its latent heat is liberated. The 

 water formed by condensation dissolves also a portion of the gas, 

 and this solution produces additional heat. In this manner an 

 extreme depression of temperature is prevented, but it is practi- 

 cable, at the same time, to maintain a lower temperature in the 

 condenser than exists in that of the steam engine. It must be 

 observed, however, that owing to the very low boiling-point of 

 the solution it is not generally practicable to reduce the pressure 

 in the condenser below half an atmosphere. 



The advantages here attributed to ammoniacal gas belong 

 also, more or less, to the vapors of many liquids more volatile 

 than water; as, for instance, ether and chloroform. Engines 

 have therefore been constructed in which these vapors have been 

 employed to produce motion by being used alone, or in combina- 

 tion with steam. The economy of using the heat of exhaust 



