64 THE ATMOSPHERE. 



sucking the poison from a wound. When the lips are 

 applied to a wound, the effort is to remove the pressure 

 of the air from its surface, that the internal pressure 

 may predominate, and force out the exposed fluids, 

 thereby carrying off the poison that may have been 

 mixed with them. 



FIRE-ENGINE. 



The greatest improvement ever made in this machine, 

 was in the addition of the airchamber. No additional 

 force is gained by its use, nor is its mechanical efficacy 

 increased; for the force requisite to compel the water 

 to enter this chamber, is exactly equal to the elasticity 

 of the compressed air. It is to be considered as a mag- 

 azine or storehouse of power, which acts uniformly upon- 

 the fluid, and forces it out in a continued stream. Be- 

 sides this, there is an absolute saving of the fluid; for 

 in every depression of the piston of the common force 

 pump, the water being thrown by jerks, a consider- 

 able quantity must fall short of the intended object. la 

 the oldfashioned engine, the water could only be thrown 

 by jerks, as the piston was forced down upon the fluid. 

 It is on this principle of compressed air, that the 

 water is raised from the Schuylkill, to supply the city 

 of Philadelphia. It is directed by the power of ma- 

 dhinery into the airchamber, whence it is forced upward 

 to a great height on the hill, and empties into the ba- 

 sins, from which it is led to the city. 



There are numerous applications of this property of 

 the atmosphere to various artificial fountains. Some of 

 them are so formed as to contain all the air naturally 

 required for their operation. Others depend upon a 

 suitable quantity being first injected by a condensing 

 syringe. 



The difference between this syringe and an airpump 

 consists merely in the inversion of both valves, whereby 

 the action of the instrument is reversed. Each valve 

 opening downward, it is easily perceived when the pistou 

 is raised, that the air must pass downward through the 

 valve, and when it is depressed, this air not beinff 

 able to escape upwards, must he forced down through 



