MECHANICS AND PHYSICS 181 



was cleverly hidden underground just beneath the 

 altars and communicating with them. Currents of 

 hot and cold air, or streams of hot and cold water, 

 or sometimes compressed air, could be used at will. 

 All that was necessary was to light the fire on the 

 altar. This fire heated the air and the water which 

 worked the subterranean mechanism. This in turn 

 acted on the statues, doves, etc., which the people 

 then beheld moving mysteriously. The gods and 

 goddesses raised their arms to bless the crowd and shed 

 tears or poured out libations. Or again a dove, lifted 

 by the hot air, rose by itself and fell to the ground 

 (Hero, I, Pneumatica, p. 338 et seq.). It is needless 

 to dwell on this point ; the interest to us of these con- 

 structions is the degree of physical and mechanical 

 knowledge which they imply. 



In this respect the forces recognized by the Ancients 

 in the realm of physics were fire, air and gravitation, 

 and also magnetic force. 



Plato spoke of the stone which Euripides called 

 Magnetic, and which was generally called the stone 

 of Hercules, which not only attracted iron rings but 

 imparted to them its own virtue (Ion, 533 D). He 

 attributed this attraction to the following phenomenon : 

 a fluid exudes from the pores of the magnet or of the 

 amber rubbed, and as a vacuum cannot exist in 

 nature, the air rushes into the pores and its movement 

 draws objects towards the magnet or electrified body. 



As regards the air, the Ancients knew that it tended 

 to rise or descend according as it is heated or cooled, 

 and that, when compressed, it escapes with violence. 

 They also knew that if the air be sucked up from a 

 tube half plunged in water, the water rises in the tube, 

 and they explained the fact as follows : bodies are 

 superposed in order of density, at the bottom the 

 solids and liquids, above them the air, then the fire ; 

 they always tend to follow one another in this order 



