SECT, iv.] AND PROPORTIONS OF ENGINES. 167 



343. When the steam acts expansively, the velocity may be found from that 

 of an expansive engine regulated by a fly, (art. 336 to 338.) as O8 times that 

 velocity will be the proper velocity for an engine for raising water. 



Thus we found the velocity for an 8 feet stroke, in an expansive engine where 

 the steam was cut off at one-fourth of the stroke, to be 245 per minute, (art. 336.) 

 and 0-8 of 245 is 196 feet per minute. 1 



In these investigations I have not attempted to enter into those minute par- 

 ticulars which embarrass the calculation, without producing any material effect on 

 the result. 



OF THE PROPORTIONS OF AIR PUMPS AND CONDENSERS FOR STEAM ENGINES. 



344. The water used for producing steam, and for condensing it, contains a 

 considerable quantity of air, and sometimes carbonic acid and other gases. These 

 gases separate when water is boiled, and rise with the steam ; hence, were there not 

 some method provided to take the air away when the steam is condensed, the 

 cylinder of a steam engine would become filled with hot air, so as to impede, and 

 in the end resist, the pressure of the steam. 



345. To estimate, therefore, the proper size for an air pump, the quantity of 

 air or other gas contained in water should be known. 



Experiments on this subject have been made by Mr. Dalton, 2 Dr. Henry, M. 

 Saussure, and Dr. Ure. M. Saussure 3 ascertained that boiling alone was not 

 capable of freeing liquids completely from air, but that it may be done by the 

 joint action of heat and the air pump. In a steam engine both these causes 

 operate in extracting air from the water introduced into the engine. According 

 to his experiments 100 volumes of water absorb about 5 volumes of atmo- 

 spheric air. 



In an experiment made by Dr. Henry 4 on spring water, he found that it 

 afforded by boiling 4'74 per cent of gaseous matter ; of which 3'38 per cent was 

 atmospheric air, and T38 per cent carbonic acid : but as it is probable that this 

 water was fully saturated, it follows from Saussure's remarks, that a greater 

 proportion would have been obtained if it had been subjected to the combined 

 action of boiling and the air pump ; and the whole proportion of gaseous matter in 

 spring water could not be estimated at less than seven per cent. 



1 [It may be proper here to observe, that in practice it is considered that from 96 to 100 feet per 

 minute should be the maximum, since at higher velocities the buckets, &c. wear rapidly, besides 

 other disadvantages. ED.] 



2 Philos. Mag. vol. xxiv. s Annals of Philosophy for 1815, vol. vi. p. 329. 

 4 Thomson's Chemistry, vol. iii. p. 204. 



