Fluid Pressure 



97 



this tendency in each case is the pressure of the air 

 on the surface of the liquid in the outer vessel. This 

 pressure is transmitted evenly throughout the liquid 

 in all directions, as has been previously explained, and 

 at the point in the tube at the same level as the surface 

 of the liquid in the vessel, we have the two forces 

 the weight of the liquid in the tube acting downwards 

 and the transmitted pressure of the air acting upwards 

 exactly counteracting each other's effects. That this 

 is the true explanation of this phenomenon was tested 

 by Pascal. He caused a tube filled with mercury and 

 inverted as above to be carried from the foot to the 

 top of the Puy de Dome, one of the lofty peaks of the 

 Auvergne Mountains. He argued that, if the column 

 of mercury was really supported by the pressure of the 

 superincumbent air, the height of the 

 column supported at the top of the moun- 

 tain should be less than that at the bottom, 

 since there must be less air above the 

 surface of the mercury in the outer vessel 

 at the top of the mountain than at the 

 bottom, and consequently it must possess 

 less weight and exert less pressure. Great 

 was his satisfaction when it was reported to 

 him that the mercury behaved exactly as 

 he had expected, the height of the column 

 supported becoming gradually less as the 

 apparatus was carried higher. We may 

 not repeat this classic experiment with 

 convenience, but we may easily investi- 

 gate the effect of removing some of the 

 air from the outer vessel by performing 



EXPERIMENT 34. Fit up a piece of 

 similar to that illustrated in Fig. 29. J 



H. D. S. 



Fig. 29. 



apparatus 

 is a wide- 



