422 Hydrodynamics. 



the most complete vacuum possible, with the least expense of 

 steam, it was necessary that the cylinder should be kept con- 

 stantly as hot as the steam itself, and that the injection of cold 

 water should take place in a separate vessel which he called the 

 condenser, and whose communication with the cylinder was sud- 

 denly opened at the moment of the injection. Indeed after what 

 is now known respecting the equilibrium of fluids, it is manifest 

 that, if the air be exhausted from the condenser, the steam from 

 the cylinder will enter it, on account of its own elasticity, the 

 instant a communication is opened ; and an injection of wa- 

 ter made at this instant, will precipitate not only the steam 

 actually in the condenser but also on the same principle, 

 all the steam contained in the cylinder, which, rushing into 

 the vacuum, continually formed by precipitation in the conden- 

 ser, is converted almost instantaneously into water. It only 

 remains, then, to remove this water and disengage the air, in 

 order to preserve always a vacuum in the condenser. Watt 

 constructed a pump in such a manner as to be moved by the 

 engine itself and which played continually in a tube void of air, 

 the lower part being immersed in the water of the condenser. 

 Finally, the condition of keeping the cylinder hot could not be 

 fulfilled while there was a free admission of atmospheric air to 

 the interior of its upper surface, which in the apparatus of New- 

 comen, caused (he piston to descend ; especially, since in order 

 to prevent the passage of the steam between the cylinder and 

 piston, i he latter was ordinarily covered with a stratum of cold 

 water which kept the interior of the cylinder wet. Watt con- 

 ceived the bold and ingenious idea of dispensing entirely with the 

 pressure of the atmosphere, and making the piston descend by 

 the force of steam alone, by introducing it alternately above and 

 below, and causing at the same time a vacuum in each case in 

 the manner already described. Then he enclosed the rod of 

 his piston in a collar of leather to prevent all access of air to 

 the interior of the cylinder; and employing steam of an elas- 

 ticity equal or even a little superior to the pressure of the at- 

 mosphere, he obtained alternately above and below the piston a 

 force equal, or a little superior, to the atmospheric pressure. He 

 was then able, by substituting stiff rods for the chains AP, A'P 1 , 

 to produce a force in both directions ; whereas, in Newcomen's 

 engine, the time during which the piston was ascending in the cy- 



