AND THE ATMOSPHERE. 221 



different from each other. Nothing, perhaps, 

 can more distinctly prove the differences which 

 exists in the principles on which the pheno- 

 mena of each depend, than the change which 

 takes place in those phenomena, either when a 

 liquid is converted into a fluid, or when a fluid 

 is converted into a solid. If a portion of 

 water, for example, is converted, by the pro- 

 cess of refrigeration, into a solid fprm, and 

 consolidated into ice; instead of pressing 

 equally in every direction, it presses in a par- 

 ticular one only ; the weight of the whole mass 

 is concentrated into one point at the bottom, 

 which point is called the centre of gravity. If 

 a block of lead, of any dimensions, say ten feet 

 square, be divided by ten lines, and immersed 

 in any medium rarer than itself; the pressure 

 of the lead, instead of extending, like the pres- 

 sure of water, equally in every direction, will 

 be entirely limited and confined, towards the 

 centre of gravity, at the bottom ; and the 

 weight, or pressure downwards, will then be 

 equal to the sum total of all the parts ; but if 

 the solid lead be liquified, it will, like the ice, 

 after it has been melted, press like other liquids, 

 equally every way. 



The observations which I have made on 

 the state of equilibrium, in which the dif- 

 ferent particles of fluids, of equal densities, 

 exist, with relation to each other, more especi- 



