232 DISCOURSE ON THE STUDY 



a portion of its weight equal to that of the liquid it 

 displaces. It seems very astonishing, after this, that 

 it should not have been at once concluded that the 

 weight thus said to be lost is only counteracted by 

 the upward pressure of the liquid, and that, there- 

 fore, a portion of any liquid, surrounded on all sides 

 by a liquid of tjie same kind, does really exert its 

 weight in keeping its place. Yet the prejudice that 

 " liquids do not gravitate in their natural place" 

 kept its ground, and was only dispelled with the 

 mass of error and absurdity which the introduction 

 of a rational and experimental philosophy by Galileo 

 swept away. I * 



(250.) The hydrostatical law of the equal pres- 

 sure of liquids in all directions, with its train of 

 curious and important consequences, is an imme- 

 diate conclusion from the perfect mobility of their 

 parts among one another, in consequence of which 

 each of them tends to recede from an excess of 

 pressure on one side, and thus bears upon the rest, 

 and distributes the pressure among its neighbours. 

 In this form it was laid down by Newton, and has 

 proved one of the most useful and fertile principles 

 of physico-mathematical reasoning on the equi- 

 librium of fluid masses, as affording a means of 

 tracing the action of a force applied at any point 

 of a liquid through its whole extent. It applies, 

 too, without any modification, to expansible fluids 

 as well as to liquids; and, in the applications of 

 geometry to this subject, enables us to dispense 

 with any minute and intricate enquiries as to the 

 mode in which individual particles act on each other. 



(251.) In a practical point of view, this law is 



