210 PHYSIOLOGICAL PHYSICS. [Chap. xx. 



fluid must be maintained, for instance, by a quantity 

 of water flowing in above constantly equal to the 

 quantity flowing out below. If, on the other hand, 

 the supply be not maintained, and the level be allowed 

 to fall, the outflow will at once diminish pari passu. 

 By the arrangement known as Marriotte's bottle, 

 however, a uniform outflow is maintained without 

 the need of maintaining the lowel of the fluid in the 

 bottle. Fig. 101 represents such a bottle. In 

 one side at the lower part is an exit 

 tube. The mouth is closed with a cork 

 pierced by a tube, both tightly fitted. 

 The tube dips down a considerable 

 way into the fluid. If the bottle and 

 the tube be full of water, the surface 

 of the water in the bottle will bear a 

 pressure equal to the atmospheric pres- 

 sure and the weight of the column of 



Fig. 101. Mar- ,, , , ,, 



riotte's Bottle, water standing m the tube above the 

 surface of the water in the bottle. If, 

 now, b be opened, and the water be allowed to flow 

 out till it stands at the same level in bottle and tube, 

 then the water in the bottle will be at atmospheric 

 pressure. At 5, accordingly, the water is pressed out- 

 wards by a force equal to that of the atmosphere + 

 the weight of the liquid column, whose height 

 is from b to the surface, and whose base is re- 

 presented by the dotted line at b ; the water is 

 also being pressed inwards by atmospheric pressure ; 

 the pressure outwards being the greater, the water 

 flows out. But, if the water be allowed to flow out 

 till all of it has passed down out of the tube a, and air 

 bubbles have begun to rise up from the tube a through 

 the water to the upper part of the bottle, then, a pressure 

 equal to that of a column of water whose height is 

 the distance from the lower part of the tube a to 

 the surface of the liquid has been removed from 



