GIN" THE CONSERVATION OF FORCE. 341 



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 KK '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 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 motive powers formerly 

 known, is that it is not restricted 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 mov- 

 able, as is the case with steam-ships and locomotives. 

 By means of these machines we can develope motive 

 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 independently of the direction of the 

 wind, and, successfully resisting storms which would drive 



