ON PNEUMATIC MACHINES. 267 



from it than if it were condensed in the usual manner as soon as its ad- 

 mission 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 ex- 

 tricated from the water during the process of boiling. If the water em- 

 ployed has been obtained from deep wells or mines, it contains more air 

 than*iisual, and ought to be exposed for some time in an open reservoir be- 

 fore it is used ; for it appears that the quantity of air, which can be con^- 

 tained in water, is nearly in proportion to the pressure to which it is sub- 

 jected. The admission of the steam into the cylinder is regulated by the 

 action of a double revolving pendulum. The piston is preserved in a situ- 

 ation very nearly vertical by means of a moveable parallelogram, fixed on 

 the beam, which corrects its curvilinear motion by a contrary curvature. 

 In the old engines, a chain working on an arch was sufficient, because 

 there was no thrust upwards. When a rotatory motion is required, it may 

 be obtained either by means of a crank, or of a sun and planet wheel,* with 

 the assistance of a fly wheel ; this machinery is generally applied to the 

 opposite end of the beam ; but it is sometimes immediately connected with 

 the piston, and the beam is not employed. The cylinder is usually inclosed 

 within a case, and the interval is filled with steam, which serves to confine 

 the heat very effectually. (Plate XXIV. Fig. 337.) 



The steam engines of Messrs. Boulton and Watt are said to save three 

 fourths of the fuel formerly used ; and it appears that only one fourth of 

 the whole force of the steam is wasted. Such a machine, with a thirty 

 inch cylinder, performs the work of 120 horses, working 8 hours each in 

 the day. 



When the water producing the condensation is to be raised from a great 

 depth, a considerable force is sometimes lost in pumping it up. Hence 

 Mr. Trevithick t has attempted, as Mr. Watt had indeed long before pro- 

 posed, to avoid entirely the necessity of condensation, by employing steam 

 at a very high temperature, and allowing it to escape, when its elasticity is 

 so reduced by expansion, as only to equal that of the atmosphere : the air 

 pump is also unnecessary in this construction, and for a small machine, it 

 may perhaps succeed tolerably well. But there must always be a very 

 considerable loss of steam, and although the expense of fuel may not be 

 increased quite in the same proportion as the elasticity of the steam, yet 

 the difference is probably inconsiderable. A great number of less essential 

 alterations have also been made in Mr. Watt's arrangements by various 

 engineers, but they have generally been calculated either for obtaining some 

 subordinate purpose of convenience, or for imposing on the public by a 

 fallacious appearance of novelty. (Plate XXIV. Fig. 338.) 



The force of steam, or of heated vapour, is probably also the immediate 

 



* After the expiry of Wasbrough's patent for the crank, the sun and planet wheel 

 was discontinued in Watt's engines, and is now never used. 



t Repertory of Arts, vol. iv. 



