498 NATURE^ TEACHINGS. 



known by the name of the Sea-squirt, and with very good 

 reasons. 



As a rule, it keeps up a rotation of tentacles, such as is 

 shown in the illustration, acting exactly on the principle of 

 the Turbine Pump, and drawing in and discharging water with 

 a power that is perfectly astonishing in so small a being. 

 Beside this, it has the power of flinging out at once the whole 

 of its watery contents, and any one who has incautiously 

 handled a mass of Ascidians, and been drenched by them, can 

 answer with more truth than satisfaction as to the water- 

 absorbing power of the Turbine. 



Then the Ascidian can do what the Turbine cannot do. In 

 the Turbine the water which is taken in must necessarily be 



TURBINK PUMP. 



ejected in equal proportions. With the Ascidian the same 

 thing takes place, but with the additional power of ejecting all 

 the contained water, and then beginning afresh. 



There is now no doubt that the Circular or the Turbine 

 Pump is the most powerful in such cases as emptying mines of 

 the water which, in spite of all precautions, will make its way 

 in, and destroy the labours of the miners. But I merely wish 

 to carry out the object of this work by remarking that the 

 invaluable Turbine Pump is only a very inferior copy of a 

 natural pump, which existed, as far as we know, centuries 

 before Man could find his place upon this earth. 



The Spiral. 



In an early portion of this work the Spiral or Screw was 

 touched upon, mostly in connection with the propulsion of 



