296 ON THE CONSERVATION OF FORCE. 



globe cools it will be drawn up towards n. In both cases 

 liquid is raised, and work thereby produced. 



The same experiment is continuously repeated on the largest 

 scale in steam-engines, though, in order to keep iip a continual 

 disengagement of compressed gases from the boiler, the air in 

 the globe in Fig. 44, which would soon reach the maximum of 

 its expansion, is replaced by water, which is gradually changed 

 into steam by the application of heat. But steam, so long as it 

 remains as such, is an elastic gas which endeavours to expand 

 exactly like atmospheric air. And instead of the column of 

 liquid which was raised in our last experiment, the machine is 

 caused to drive a solid piston which imparts its motion to other 

 parts of the machine. Fig. 45 represents a front view of the 

 working parts of a high-pressure engine, and Fig. 46 a section. 

 The boiler in which steam is generated is not represented ; the 

 steam passes through the tube z z, Fig. 46, to the cylinder A A, 

 in which moves a tightly fitting piston C. The parts between 

 the tube z z and the cylinder A A, that is the slide valve in the 

 valve-chest K K, and the two tubes d and e allow the steam to 

 pass first below and then above the piston, while at the same 

 time the steam has free exit from the other half of the cylinder. 

 When the steam passes under the piston, it forces it upward ; 

 when the piston has reached the top of its course the position 

 of the valve in K K changes, and the steam passes above the 

 piston and forces it down again. The piston-rod acts by means 

 of the connecting-rod P, on the crank Q of the fly-wheel X and 

 sets this in motion. By means of the rod s, the motion of the 

 tod regulates the opening and closing of the valve. But we 

 need not here enter into those mechanical arrangements, how- 

 ever ingeniously they have been devised. We are only interested 

 in the manner in which heat produces elastic vapour, and how 

 this vapour, in its endeavour to expand, is compelled to move 

 the solid parts of the machine, and furnish work. 



You all know how powerful and varied are the effects of 

 which steam-engines are capable ; with them has really begun 

 the great development of industry which has characterised our 

 century before all others. Its most essential superiority over 



