EVAPORATION. 261 



a suitable boiler. The power of the engine is of course 

 proportioned to the size or area of the piston, on which 

 the steam acts with a force according to its density, of 

 from 15 to 100 or more pounds to each square inch. In 

 some of the mines in Cornwall, England, there are cylin- 

 ders and pistons of more than 70 inches in diameter, on 

 which the pressure of the steam equals the effort of 600 

 horses. So extensively is this engine used in England, 

 that it is an important source of her national power. 

 There one engine is often seen stretching its long arms 

 over an extensive manufactory, and guidir^ all its com- 

 plicated movements seemingly with the precision and 

 more than the precision of intellect. In one part of the 

 building it is keeping thousands of spinning-wheels in 

 motion, while in another it is carding the material, and 

 in a third weaving the cloth. In like manner, one steam- 

 engine in a great brewery may be seen at the same time 

 grinding the malt, pulling up supplies of all kinds from 

 wagons in various situations, pumping cold water into 

 some of the coppers, sending the boiling wort from 

 others into lofty cooling-pans, over which it is turning 

 the fans, and in a word, performing the offices of a hun- 

 dred hands. It is not strange that the eulogist of Watt, 

 the great improver of the steam engine in England, 

 should say, ' it is the steam engine which has fought our 

 battles and enabled us to come off victorious in the late 

 tremendous contest.' Had it not been for this mighty 

 engine, perhaps England herself would have been com- 

 pelled to bend under the power of Napoleon, aided as he 

 was at one time by the power of almost all Europe. 



Water, if not confined, slowly evaporates and incor- 

 porates with the atmosphere at any temperature above 

 the freezing point. And it is highly probable that even 

 freezing does not wholly put a stop to this process. Ice 

 appears gradually to waste away even when the surround- 

 ing air is at a temperature far below the freezing point. 

 When water is rarefied to a certain degree, it becomes 

 lighter than the surrounding air, and consequently rises> 

 although its presence in the air is not generally perceiv- 

 ed. The air can contain a certain quantity of moisture 

 so dissolved as to be perfectly invisible. In this state it 



VOL. I. NO. XI. 23* 



