SECTION VII. 



OF THE PROPORTIONS, AND THE CONSTRUCTION, OF THE PARTS OF 



STEAM ENGINES. 



431. THE steam engine has hitherto been studied as a whole ; but in order to 

 become more perfectly acquainted with its nature, we must dissect it, and study it 

 in parts. This forms the object of the present section. Some of these parts have 

 to be considered only as far as strength is concerned, as beams, shafts, cross-heads, 

 &c. ; others in respect to the motions they are to produce, as the parallel motion, 

 the eccentric motion, &c. ; others depend on the combination of moveable parts 

 with accuracy of workmanship, as pistons, valves, &c. besides the modes of con- 

 structing joints, &c. According to the dependence of these parts on one another, it 

 seems desirable to treat them in the order of valves, pistons, stuffing boxes, hand 

 gear, piston guides, parallel motion, strength of parts, (as beams, cranks, wheel- 

 arms, gudgeons, and teeth of wheels, cross-heads and frames, shafts and 

 journals, piston rods, connecting rods, and parallel motion rods, cylinders, pipes, 

 and boilers,) and joining pipes. 



OF COCKS AND VALVES. 



432. Under the head " Cocks and Valves " may be included all those methods 

 which may be found useful for opening and closing passages for steam. It is of 

 some advantage, in discussing their respective merits, to class them ; and the most 

 simple method of doing this seems to be by the motion which opens them. 

 In this way, our arrangement will be as follows : 



