4 2 



ELEMENTS OF SCIENCE 



FIG. 3. 



a pile of them as a very slight disturbance will cause to 

 fall. 



In very stable structures, like the ancient Egyptian 

 buildings, such pressure was most amply provided for, 

 and an equilibrium of the most stable kind produced. 



This was the case above all with the Pyramids, and to 

 a less degree in such temples as those of Philse and Karnac 

 so impressive from the superfluous strength of their 

 many rows of close-set, massive columns. In Grecian 



buildings we meet with the 

 same secure repose, but in 

 greater delicacy of build. In 

 the arch and the dome, how- 

 ever, and still more in pointed 

 architecture, the conflict of 

 stones which tend to fall in 

 different directions, produces 

 (by the neutralising of each 

 other's thrusts) a different 

 kind of equilibrium and one 

 of a less stable character. 



As every one knows, sub- 

 stances of the same size of 

 various kinds may be very 

 different in weight ; as we see 



in a cube made of cork and another of precisely the 

 same size made of lead, or two glass vessels of the same 

 size, one filled with water and the other with quicksilver 



or " mercury." 



Heavier bodies are said to be more dense than lighter 

 bodies of the same size. Thus experiment shows us that 

 the density of Mercury is 13.6 times that of water. 

 Similarly the density of lead is much greater than that 

 of wood. Substances also differ in the extent to which 



