A HISTORY OF SCIENCE 



serving a most useful purpose for the priests, who alone, 

 we may assume, were in the secret. There were two 

 methods by which this apparatus was worked. In 

 one the heated air pressed on the water in a close re- 

 tort connected with the altar, forcing water out of the 

 retort into a bucket, which by its weight applied a 

 force through pulleys and ropes that turned the stand- 

 ards on which the temple doors revolved. When the 

 fire died down the air contracted, the water was si- 

 phoned back from the bucket, which, being thus light- 

 ened, let the doors close again through the action of an 

 ordinary weight. The other method was a slight mod- 

 ification, in which the retort of water was dispensed 

 with and a leather sack like a large football substitued. 

 The ropes and pulleys were connected with this sack, 

 which exerted a pull when the hot air expanded, and 

 which collapsed and thus relaxed its strain when the 

 air cooled. A glance at the illustrations taken from 

 Hero's book will make the details clear. 



Other mechanisms utilized a somewhat different 

 combination of weights, pulleys, and siphons, operated 

 by the expansive power of air, unheated but under 

 pressure, such pressure being applied with a force- 

 pump, or by the weight of water running into a closed 

 receptacle. One such mechanism gives us a constant 

 jet of water or perpetual fountain. Another curious 

 application of the principle furnishes us with an elab- 

 orate toy, consisting of a group of birds which alter- 

 nately whistle or are silent, while an owl seated on a 

 neighboring perch turns towards the birds when their 

 song begins and away from them when it ends. The 

 "singing" of the birds, it must be explained, is pro- 



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