CHAP, v.] THE STEAMENGINE. 389 



CHAPTER V. 



THE STEAM-ENGINE. 



I. THE MOTIVE POWER OF STEAM. 



THE ancients were acquainted with the elastic force of steam, and 

 without having any very clear or precise notions of its physical 

 properties, they endeavoured to avail themselves of it. 



For this purpose Hero of Alexandria invented the machine to 

 which he gave the name of eolipyle, as well as other apparatus in which 

 the action of compressed or rarefied air was called into play. We 

 shall see, in fact, that the movement of the eolipyle was simply caused 

 by the expansive force of the steam, though working in an entirely 

 different manner from that of a modern steam-engine. 



It consisted of a pot or boiler, partly filled with water, placed on 

 the fire, and closed by a lid. Over this was fixed a hollow bent tube, 

 with a tap, which supported and communicated with the inside of 

 a hollow metallic sphere, which "was also supported at the opposite 

 extremity of the diameter by another tube not communicating with 

 the inside. The sphere was movable about this axis of support. 

 Two other hollow bent tubes projected from the surface of the sphere 

 in a direction perpendicular to the axis of rotation. With this 

 explanation we can easily understand the motive power of the steam 

 in this little apparatus. The tap is opened, the steam rushes from the 

 boiler to the tube and fills the sphere. If this were entirely closed, it 

 would remain motionless, but the steam which exerts the same pres- 

 sure on all points of the inner surface of the sphere, finding two 

 openings, escapes with a noise as it condenses in the air ; the reaction 

 which would produce equilibrium if the sphere were entirely closed, 

 exerts a force in the contrary direction; and the sphere revolves 



