388 



STEAM ENGINE. 



Newcomen, commonly called the Atmospheric 

 engine. This engine is commonly used to raise 

 water, the pump rods being substituted instead of 

 the weight W. 



Instead of injecting the cold water into the cylin- 

 der, Watt formed a communication between the 

 bottom of it and another hollow vessel, M, fig. 2, 

 called the condenser. Into this vessel a shower of 

 cold water is continually flowing through a rose, 

 the steam is condensed in consequence, and the 

 cold water, together with the condensed steam, are 

 taken out of the condenser by means of a pump N 

 worked by the engine itself, so that the condenser 

 is continually cleared. 



In order to keep the piston parallel it was usual 

 to attach the top of it to a chain, which adapted 

 itself to a wooden arch at the end of the beam, but 

 this has, in general, been abandoned for the more 

 perfect contrivance for the same purpose, called the 

 parallel motion, to be described hereafter. 



It is not difficult to see that a very considerable 

 addition of power could be derived from this en- 

 gine were the pressure of the atmosphere taken off 

 the top of the piston when it is in the act of rising. 

 This is accomplished in the following manner. The 

 cylinder is closed at the top, by a cover, as shown 

 in fig. 3. Through the centre of the cover a cir- 

 cular opening is made to permit the piston rod to 

 rise and fall, and a small metallic box is placed upon 

 it, containing hemp, which envelopes the piston rod, 

 and prevents the passage of air or steam by the 

 opening. The upright tube a b communicates 

 with the cylinder, both at the top and bottom, and 

 also communicates with the steam boiler by a pipe 

 I, as likewise with the condenser by means of a 

 pipe D D. The upright pipe a b contains a rod, 

 passing up through it, and movable up and down 

 through a hemp stuffed box in the top, similar 



to that for the piston rod in the cylinder cover. 

 This upright rod, or spindle, carries two pistons O O, 

 which move up and down past the openings into 

 the cylinder at the top and bottom. When the 

 pistons are below the openings, or port holes, as they 

 are called, as shown in fig. 3, then it is plain that 

 there will be a free communication between the 

 bottom of the cylinder, and the steam from the 

 boiler, and at the same time a free communication 



between the top of the cylinder and the con- 

 denser. When the pistons are raised above the 

 openings or port holes, then the case is reversed, 

 the steam is cut off from the bottom of the cylinder, 

 and admitted to the top, while the condenser is 

 opened to the bottom and closed to the top. On 

 whichever side of the piston the steam acts, it will 

 move the piston in the direction of its own motion, 

 that is, when the steam issues in at the top of the 

 cylinder the piston will descend to the bottom, and 

 when admitted at the bottom, the piston will rise 

 to the top, there being nothing but its own friction 

 to oppose its ascent or descent, as the moment that 

 the steam is opened to one side, the ether side is 

 put into a state of vacuo, by being opened to the 

 condenser. Such is the principle of the double act- 

 ing condensing engine of Watt. If the steam be 

 used of higher pressure than 14 Ibs. to the square 

 inch, and cut off before the piston has moved 

 through the whole length of the cylinder, being 

 then allowed to expand itself, the engine is of the 

 expansive kind. If there be no condenser at all, 

 and the steam be used of very high pressure, and 

 instead of being condensed be allowed to escape into 

 the open air, after it has moved the piston, then the 

 engine is of the high pressure kind. 



Having now laid down the general principles on 

 which condensing and high pressure engines.act, we 

 will now describe them as actually constructed in 

 all their parts, beginning with the high pressure 

 engine, its arrangements being the most simple. 



In the accompanying wood cuts, figs. 4 and 5 

 show a high pressure steam engine of the simplest 

 construction. Fig. 4 is a side elevation, in which 

 some of the parts are drawn in section, and fig. 5 is 

 a ground plan. The same letters of reference are 

 used for both figures. The hollow cylinder a y is 

 made of cast-iron, and is accurately bored; the 

 piston b is also made of cast-iron, accurately fitted 

 to the cylinder. A flat groove is formed round 

 the circumference of the piston, which is packed 

 full of plaited hemp yarn, in order to prevent the 

 steam in the cylinder from passing between the 

 side of the cylinder and the piston. The piston 

 rod c, which is made of malleable iron, is firmly 

 fixed to the piston, and turned and polished, so 

 that it will slide easily through the stuffing box, 

 which forms part of one of the covers of the cylinder. 

 In order to prevent the escape of steam, the stuf- 

 fing box is packed with hemp. The cross head, or 

 part fixed upon the end of the piston rod, which is 

 outside of the cylinder, is connected to the crank/ 

 by means of the rod g g; one end of this rod works 

 upon two turned parts on the cross head, and its 

 other end works upon a pin fixed into the crank. 

 The construction of the brass bushes into which 

 the crank pin and the centres on the cross head 

 work, are fully shown in the figures ; and it may 

 be remarked that the crank shaft, and other work- 

 ing parts of the engine, work into similar brass 

 bushes. On the shaft h h, the fly wheel i i, is 

 firmly fixed; for small engines this shaft and its 

 crank are cast in one piece, but in large ones they 

 are fixed together by bolts or otherwise. In 

 order to show the parts more fully, the crank is 

 drawn in a position perpendicular to the horizon in 

 fig. 4, while its position is a horizontal one in fig. 5. 

 The plummet block I, into which the crank shaft 

 works, is in ordinary cases bolted upon a stone in 

 the wall, and an opening is left through the wall 

 round the block. The part k of the crank shaft is 

 made so that a shaft or a wheel fixed on it m*y put 



