CAPTIVE MOLECULES 



steam to enter the cylinder where it can act on a piston, 

 to which it gives the thrust that is transmitted to an 

 external mechanism by means of a rod attached to the 

 piston. When the piston has been driven to the end of 

 the desired thrust, the valve is opened automatically, 

 permitting the steam to escape, thus producing a vac- 

 uum, and insuring the return thrust of the piston, which 

 is further facilitated, ordinarily, by the admission of 

 steam to the other side of the piston. Practical opera- 

 tion of this mechanism is familiar to everyone, though 

 the marvel of its power and efficiency seems none the 

 less because of its familiarity. 



It is not too much to say that this relatively simple 

 device, in its first general application, marked one of 

 the most important turning points in the history of 

 civilization. To its influence, more than to any other 

 single cause, must be ascribed the revolutionary change 

 that came over the character of practical life in the 

 nineteenth century. From prehistoric times till well 

 toward the close of the eighteenth century, there was 

 scarcely any important change in carrying out the 

 world's work. And in the few generations that have 

 since elapsed, the entire aspect of the mechanical world 

 has been changed, the working efficiency of the individ- 

 ual has been largely increased ; mechanical tasks have 

 become easy which hitherto were scarcely within the 

 range of human capacity. 



Before we go on to the detailed study of the machine 

 which has produced these remarkable results, it is de- 

 sirable to make inquiry as to the historical development 

 of so important an invention. 



VOL. vn. 6 [ 8l 1 



