252 



WELLS'S NATURAL PHILOSOPHY. 



How is steam 576. Steam is rendered useful for mechan- 



madc available • ^ • i ^ • , i , • 



for mechanical ical purposes sunplj Dj its prcssure, or elastic 



purposes? ^^^^^_ 



Steam can not, like wind and -vrater, be made to act advantageously by ita 



Fig. SIO. 



impulse in the open air, because the momentum of so 

 light a fluid, milcss generated in vast quantities, would 

 be inconsidi,Table. The first attempts, however, to 

 employ steam as a moving power, consisted in direct- 

 ing a current of steam from the mouth of a tube against 

 the floats or vanes of a revolving wheel. 



A machine of this kind, invented more than 2,000 

 years ago by Hero of Alexandria, is represented in 

 Fig. 210. It consists of a small hollow sphere, fur- 

 nished with arms at right angles to its axis, and v.iioso 

 ends are bent iu opposite directions. The sphere 

 is suspended between two columns, bent and pointed 

 at their extremities, as represented in the figure : .one 

 of these is hollow, and conveys steam from the boiler 

 below, into the sphere; and the escape of the vapor 

 from the small tubes, by the reaction, produces a rotary motion. 



In ordpr to render the presstire of steam practically availa- 

 ble in machinery, it is necessary that it should be confined 

 within a cavity which is air-tight, and so constructed that its 

 dimensions or capacity can be enlarged or diminished without 

 impairing its tightness. When the steam enters such a ves- 

 sel, its elastic force pressing agrjnst some movable part, causes 

 it to recede before it, and from this movable part motion is communicated to 

 machinery. 



„ , The practical arrangement bv which such a 



now are these , . ,.,,.,". , ,, 



conditions at- result IS accomplished is by having a hollow 



tained ? cylinder, A B, Fig. 211, with a movable piston, 



D, accurately fitted to its cavity. When steam under pressure 



in a boiler is admitted into the cylinder below the piston, it ■ 



expands, and acting upon the under surface of the piston, 



causes it to rise, lifting the piston-rod along with it. 



Suppose, as in Fig. 212, the cylinder to be connented at the 



bottom or side with a pipe, E, opening into a steam boiler, and 



on the other side with a pipe, B, terminating in a vessel of 



cold water. Suppose the valve in R to bo open, and that 



in B to be shut; steam then passing into the cylinder from 



the boiler will force the piston up to the top of the cylinder. 



Let the valve in E, then be shut, and the valve in B be 



opened; the steam contained in the cylinder will pass out ' 



of the pipe B, and coming in contact with cold water, in 



the vessel connected with it, will bo condensed, and a vacuum fona* 



To render the 

 pressure of 

 stearu availa- 

 ble in machin- 

 ery, what con- 

 ditions are 

 necessary ? 



Fia 211. 



