348 LECTURE XXIX. 



The cylinder of Beighton's machine is ne_cessarily much cooled by the ad- 

 mission of the jet, and by exposure to the air. Mr. Watt has avoided this in- 

 convenience by performing the condensation in a separate vessel, into which 

 a small jet is flowing without intermission ; and by introducing the steam 

 alternately above and below the piston, the external air is wholly excluded ; 

 the piston rod working in a collar of leathers, so that the machine has a 

 double action, somewhat resembling that of Lahire's double pump; and the 

 stroke being equally effectual in each direction, the same cylinder, by means 

 of an increased quantity of steam, performs twice as much work as in the 

 common engine. We might also employ, if we thought proper, a lower 

 temperature than that at which water usually boils, and work in this man- 

 ner with a smaller quantity of steam; but there would be some difficulty in 

 completely preventing the insinuation of the common air. On the other 

 hand, we may raise the fire so as to furnish steam at 220° or more, and thus 

 obtain a power somewhat greater than that of the atmospheric pressure; and 

 this is found to be the most advantageous mode of working the engine; but 

 the excess of the force above the atmospheric pressure cannot be greater than 

 that which is equivalent to the column of water descending to supply the 

 boiler, since the water could not be regularly admitted in opposition to such 

 a pressure. The steam might also be allowed to expand itself within the cy- 

 linder for some time after its admission, and in this manner it appears from 

 calculation that much more force might be obtained from it than if it were 

 condensed in the usual manner as soon as its admission ceases; but the 

 force of steam thus expanding is much diminished by the cold which always 

 accompanies such an expansion, and this method would be liable to several 

 other practical inconveniences. 



The peculiarities of Mr. Watt's construction require also some other ad- 

 ditional arrangements; thus, it is necessary to have a pump, to raise not only 

 the water out of the condenser, but also the air, which is always extricated 

 from the water during the process of boiling. If the water employed has 

 been obtained from deep wells or mines, it contains more air than usual, and 

 ought to be exposed for some time in an open reservoir before it is used ; for it ap- 

 vpears that the quantity of air, which can be contained in water, is nearly in propor- 

 ■*^ion to the pressure to which it is subjected. The admission of the steam into 



