58 



THE REALITIES OF MODERN SCIENCE 



of the gas for which we wish to know the pressure. 

 By Boyle's Law, then, we may calculate the pressure 

 from the new volume of the air in the short arm. 



In describing this experiment on Boyle's Law and 

 its application to the construction of a manometer we 

 have tacitly assumed that the temperature has been 

 kept constant. As we shall see later, and as the 

 reader realizes, changes in temperature will cause 

 changes in the volume which the air occupies, apart 

 from any change in pressure. We have also assumed 

 that the tube is everywhere the same size. 



If a tube like that of Fig. 5 is used the same experi- 

 ment may be performed with identical results. Of 

 course, more mercury needs to be added 

 to produce the same difference of level. 

 It is to Pascal that we owe the statement 

 of the general principle which underlies 

 this phenomenon. To understand it we 

 distinguish between force and pressure, 

 defining pressure 1 as the force per unit 

 area. 



Now Pascal's Law states that fluids at 

 rest transmit pressure equally in every 

 Is not this obviously the condition which 

 must be met if a fluid is to be at rest ? If a particle 

 is to be at rest it must sustain pressures from the 

 liquid about it which are the same in all directions, 

 for if more pressure is transmitted to it from one side 

 than the other it will move. Conversely, the particle 



1 If the pressure is the same on every square inch of cross-sec- 

 tional area then the total force exerted is the product of the area 

 and the pressure. 



FIG. 5. 



direction. 



