172 



FERGUSON'S LECTURES. 



LECT. 

 V. 



the half of a beam, p being the 

 centre upon which it turns while 

 the piston rod is attached to 

 its end q., by connection at 

 r with the lower end of the 

 intermediate piece between q 

 and r ; in order to produce a 

 vertical right lined motion in 

 this piston rod, another piece s is introduced, equal in length to 

 the first intermediate piece, and hung so as to turn upon pivots 

 fixed to the beam between its centre and extreme end as at t ; 

 to the lower part of this second piece, the secondary beam or 

 radius rod v u is attached, so that it may turn upon a fixed pivot 

 at u, while its length is equal to the distance p t, consequently 

 it will produce a vertical rectilinear motion in the lower extre- 

 mity v of the sling, or second intermediate piece s, and this is 

 communicated back to the point r, where the piston is attached, 

 by the bridle rod w having pivots working in the lower extremi- 

 ties of v and r. In this way two right lined vertical motions are 

 produced at the points v and r whenever the beam moves, con- 

 sequently, while the steam piston is attached to r, the piston rod 

 of the air pump is generally applied to v, since they both require 

 a perpendicular motion. 



The action of the high pressure engine, depends upon the 

 properly of steam to expand itself, and thus acquires a very con- 

 siderable elastic force by the addition of a given quantity of heat. 

 It may indeed, be considered as a return to the principle of 

 Brancas and the Marquis of Worcester, as in this engine no con- 

 densation is necessary, and it acts merely by the elastic or re- 

 pellent force of steam. 



The high pressure engines constructed by Messrs. Trevithick 

 differ but in a very small degree from the engine invented by Mr. 

 Watt, [n the high pressure engine the condenser is taken away, 

 and the steam, instead of being converted into water by artificial 

 cold, in a close vessel, is allowed to escape into the atmosphere 

 from one side of the piston while it is acting forcibly on the 

 others. 



The advantages of the high pressure engine over that used 

 with a condenser are, cheapness in construction, and a saving of 

 the whole expence attendant on procuring a sufficient supply of 

 condensing water, which, in some cases, is an object of con- 

 siderable importance. 



