339 



LECTURE XXIX. 



ON PNEUMATIC MACHINES. 



Pneumatic machines are such as are principally dependent, in theij 

 operation, upon the properties of elastic fluids ; they may be calculated either 

 for diminishing or increasing their density and pressure, as air pumps and 

 condensers; or for directing and applying their force, as bellows, ventilators, 

 steam engines, and guns. 



The density and pressure of the air may be diminished, or the air may 

 be perfectly or very nearly withdrawn from a given space, either by means 

 of a column of mercury, or by the air pump. The ancients sometimes 

 exhausted a vessel imperfectly by the repeated action of the mouth, and 

 preserved the rarefaction by the assistance of a stopcock. The Torri- 

 cellian vacuum, obtained by inverting a receiver filled with mercury, and 

 furnished with a descending tube at least 30 inches long, is the most perfect 

 that can be procured ; but there is generally a portion of air adhering to the 

 vessels, and mixed with the mercury, which may often be considerably di- 

 minished by agitation, but can only be completely expelled by boiling the 

 mercury for some time in the vessel and its tube, previously to their inver- 

 sion. (Plate XXIV. Fig. 324.) 



The construction of an air pump greatly resembles that of a common 

 sucking pump for raising water ; but the difference in the operation to be 

 performed requires a difference in several particular arrangements. The 

 objects are, to rarefy or exhaust the air as completely, as expeditiously, and 

 as easily, as possible. In order that the exhaustion may be complete, it is 

 necessary that no air remain in the barrel when the valve is opened, and that 

 the process be very long continued. For, supposing all the parts of an air 

 pump to be perfectly well fitted, and the exhaustion to be carried on for any 



