158 THE QUAQUAVERSUS PRESSURE OF FLUIDS 



lower towards the higher extremities of the tubes, which are supposed 

 to rise to a height considerably above the surface of the water. 



Now, since the lower extremities of the tubes may be conceived to 

 point in every possible direction, it follows, that the pressure of the 

 superincumbent fluid is also propagated in every direction. But when 

 it is required that the lower orifice should point directly downwards, 

 in order to show the upward pressure of fluids, a straight tube must 

 be employed, and the mercury which is introduced must be kept in 

 by the finger, until the height of the water above the lower surface, is 

 about fourteen times the height of the mercurial column ; for if the 

 finger be removed before the water has attained that height, the mer- 

 cury will fall out of the tube, since its weight is fourteen times greater 

 than the weight of an equal bulk of water. If the finger be continued 

 upon the orifice, until the height of the water be equal to fourteen 

 times the height of the mercury, then, on removing the finger, and 

 pouring in more water, the mercury will be seen to ascend in the 

 tube, and will continue to rise higher and higher, according to the 

 quantity of water poured in, thereby showing the upward pressure of 

 the water. 



EXPERIMENT 4. The pressure of fluids at different points of their 

 depths, may be very simply illustrated 

 in the following manner : let K be a bag 

 of leather, or some other tough and 

 flexible material, filled with mercury, 

 and attached to the extremity of a 

 glass tube zi, in such a manner, that 

 the mercury may just enter the tube 

 when the bag is held in air. 



Then, if the bag be immersed in 

 water, it is manifest that the pressure 

 of the fluid will cause it to collapse, and the mercury will ascend in 

 the tube to a certain height, corresponding to the pressure exerted by 

 the water, at the depth where the bag is placed. If the bag continue 

 to be lowered in the water, it will become more and more collapsed in 

 consequence of the increased pressure, and the mercury will ascend 

 higher and higher in the tube, and the heights to which it rises, will 

 indicate the magnitude of pressure at different depths. 



EXPERIMENT 5. There is a very simple and amusing experiment, 

 by which the propagation of pressure through fluids is illustrated, 

 called the " Cartesian Devil" from M. Descartes, the celebrated 

 French philosopher, by whom it was discovered ; it is as follows. 



