BOOK VI. 



205 



prone to motion as it revolves. And so, when the workman turns the axle 

 by the crank, the fans, the description of which I will give a little later, draw 

 in the £iir by the blow-hole, and force it through the other blow-hole which 

 leads to the conduit, and through this conduit the air penetrates into the 

 shaft. 



The one with the box-shaped casing is furnished with just the same 

 things as the drum, but the drum is far superior to the box ; for the fans so 

 fill the drum that they almost touch it on every side, and drive into the 

 conduit cdl the air that has been accumulated ; but they cannot thus fill 

 the box-shaped casing, on account of its angles, into which the air partly 

 retreats ; therefore it cannot be as useful as the drum. The kind with a 

 box -shaped casing is not only placed on the ground, but is also set up on timbers 

 like a windmill, and its axle, in place of a crank, has four sails outside, 

 like the sails of a windmill. When these are struck by the wind they turn 

 the axle, and in this way its fans — which are placed within the casing — drive 



A— Box-shaped casing placed on the ground. B— Its blow-hole. C— Its axle 



WITH FANS. D— Crank OF THE axle. E— Rods of same. F— Casing set on timbers. 



G— Sails which the axle has oxttside the casing. 



16 



