SCIENCE AND PROGRESS 



tions. The little, revolving fountains which are set 

 out upon the lawn or in the garden, and throw a 

 circular stream, perforce of the push of the water 

 as it comes from the revolving nozzles, work upon 

 exactly the same principle. 



This was about 200 B.C. The invention lay dor- 

 mant for 2100 years. Hero's steam-turbine was 

 reinvented some fifteen or twenty years ago by 

 a Swedish engineer named de Laval. Since then 

 several other forms of steam-turbines have been 

 constructed, notably that of Parsons, which is in- 

 stalled on the swift-going Turbinia. If one may 

 judge from the developments of the last ten years, 

 Hero's device, modified and improved, is likely in 

 part to drive out other forms of steam-engines. 



The modern hot-air engines were developed from 

 another ingenious device of this same Hero. On 

 a hollow altar, containing air, a fire was kindled. 

 The heated air within expanded and drove a col- 

 umn of water below into another vessel; in so 

 doing a pulley arrangement was worked and the 

 doors of the temple swung open. When the fire 

 died and the air cooled, the device worked back- 

 ward; the doors of the temple closed. It must 

 have been thought a miracle then. The priests of 

 those days were inventors and men of science. 



The list of Alexandria's great men here given is 

 but a slender detail the mention of a few names 



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