200 HELMHOLTZ 



parts of a high-pressure engine, and FIG. 98 a section. The 

 boiler in which steam is generated is not represented; the 

 steam passes through the tube z z, FIG. 98, 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 rod regulates the 

 opening and closing of the valve. But we need not here 

 enter into those mechanical arrangements, however ingeni- 

 ously 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 motive powers formerly known is that it is not re- 

 stricted to a particular place. The store of coal and the 

 small quantity of water which are the sources of its power 

 can be brought everywhere, and steam-engines can even be 

 made movable, as is the case with steam-ships and loco- 

 motives. By means of these machines we can develop mo- 

 tive power to almost an indefinite extent at any place on 

 the earth's surface, in deep mines and even on the middle 

 of the ocean ; while water and wind mills are bound to special 

 parts of the surface of the land. The locomotive transports 

 travellers and goods over the land in numbers and with a 

 speed which must have seemed an incredible fable to our 

 forefathers, who looked upon the mail-coach with its six 

 passengers in the inside, and its ten miles an hour, as an 

 enormous progress. Steam-engines traverse the ocean in? 



