THE WORK OF AIR AND WATER 



water were also but little understood by the ancients. 

 In the days of slave labor it was scarcely worth while 

 to tax man's ingenuity to invent machines, since so 

 efficient a one was provided by nature. Yet the prop- 

 erties of both air and water were studied by various 

 mechanical philosophers, at the head of whom were 

 Archimedes, whose work has already been referred to, 

 and the famous Alexandrian, Ctesibius, whose investi- 

 gations became familiar through the publications of his 

 pupil, Hero. 



Perhaps the most remarkable device invented by Ctes- 

 ibius was a fire-engine, consisting of an arrangement 

 of valves constituting a pump, and operating on the 

 principle which is still in vogue. It is known, however, 

 that the Egyptians of a much earlier period used 

 buckets having valves in their bottoms, and these per- 

 haps furnished the foundation for the idea of Ctesibius. 

 It is unnecessary to give details of this fire-engine. 

 It may be noted, however, that the principle of the lever 

 is the one employed in its operation to gain power. A 

 valve consists essentially of any simple hinged sub- 

 stance, arranged so that it may rise or fall, alternately 

 opening and closing an aperture. A mere flap of 

 leather, nailed on one edge, serves as a tolerably effec- 

 tive valve. At least one of the valves used by Ctesibius 

 was a hinged piece of smooth metal. A piston fitted 

 in a cylinder supplies suction when the lever is raised, 

 and pressure when it is compressed, alternately opening 

 the valve and closing the valve through which the water 

 enters the tube. Meantime a second valve alternating 

 with the first permits the water to enter the chamber 



