58 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK i. 



During this time, the other piston ejects through the conducting 

 pipe the water already inside. At each half turn the two pistons 

 change their functions ; that which drew up ejects and vice versd, so 

 that the pump is to a certain extent a double-action pump. The 

 double action is easily seen by examining what happens during 

 an entire rotation by comparing, for instance in Fig, 39, the respective 

 positions of the pistons and the spaces a, a', after each quarter turn. 



Perhaps the most important form of rotating pump is that known 

 as the centrifugal. There are several varieties of this system, but 

 the principle on which they all act is identical. The pump consists 

 of a circular chamber in which revolves with great rapidity a wheel 

 or fan, the arms of which curve outwards, so that all the air or water 

 contained in the chamber is driven by the so-called "centrifugal 

 force " away from the centre. The delivery pipe is therefore placed 

 at the circumference of the chamber and the fluid is sucked in at its 

 centre. It will be seen that these pumps do not produce a complete 

 vacuum, and therefore are not suitable for drawing water long dis- 

 tances ; but owing to their simplicity they are of great use for raising 

 large quantities of water a short distance, as, for example, in draining 

 marshy lands. They are generally driven by steam power. 



It only remains for us to complete all that relates to this head, to 

 speak of force-pumps, although as we have before stated, their 

 construction is by no means based on the principle of the action of 

 atmospheric pressure. 



II. FIRE-ENGINES. 



Fire-engines and pumps used for watering gardens are of the 

 kind we have defined as force pumps. 



Hand fire-engines (Fig. 41) are generally composed of two force- 

 pumps joined together, and connected with a reservoir, which is filled, 

 either by pails (and the formation of a chain of men to pass them) or 

 by pipes connecting them with the water supply of towns. They are 

 worked by a lever to which are attached the rods of the two pistons. 

 These move in contrary directions, so that the water is forced 

 continuously into the space in which the ejection piston descends 

 This space contains air, which, being compressed by the water with 

 which it is continuously supplied, exercises a pressure on the liquid ; 



