TIDES, WAVES 95 



This experiment has since been repeated with 

 globes made of other metals with like results, 

 and for a long time all liquids were regarded 

 as being absolutely incompressible, but subse- 

 quent researches have shown that liquids are 

 really slightly compressible. Some authors 

 say the compression is proportional to the 

 pressure up to a pressure of 65 atmospheres, 

 but P. G. Tait and J. Y. Buchanan have shown 

 that compressibility decreases slightly with 

 increase of pressure. Water is compressible 

 by about one twenty-thousandth of its bulk 

 under the pressure of one atmosphere. At 

 4000 fathoms the pressure would reduce the 

 bulk of 10,500 cubic feet of surface water to 

 about 10,000 cubic feet, and Tait calculated 

 that, if gravity should suddenly cease to act, 

 the surface of the oceans would immediately 

 rise 200 feet. 



Pressure at Different Depths, — The pressure 

 of the atmosphere at sea-level may be taken 

 as 15 lbs. to the square inch, which is equal 

 to the weight of 30 inches of mercury. Thirty- 

 four feet of fresh water and 33 feet of salt water 

 are equal to 30 inches of mercury, so that at 

 33 feet of depth in the sea the pressure would 

 be two atmospheres including the true atmo 

 sphere, at a depth of 66 feet three atmo- 

 spheres, at 99 feet four atmospheres, or 60 lbs. 

 to the square inch, and so on, as shown in the 



