120 THE WORLD. 



as Galileo did, that it was " Nature's abhorrence of a va- 

 cuum." It is however, effected by the pressure of the atmosphere. 

 When an open tube is dipped at one eud into the water, the li- 

 quid does not rise in the tube until the air within it is ramored by 

 exhaustion, as we have described, when immediately the fluid 

 rises, because the atmosphere without the tube, pressing upon the 

 mobile particles of the fluid, forces them up. Now it is evident 

 that if the tube was long enough, and the exhaustion perfect, the 

 pressure of the air upon the liquid outside would raise a column 

 of water just so high that the weight of this elevated water would 

 be equal to the pressure of the atmosphere upon the liquid at the 

 bore or orifice of the tube. The column of water which can be 

 thus raised or sustained, is generally about 33 feet high; and as 

 such a column when its area is one square inch, weighs about 15 

 pounds, we infer that the atmosphere presses upon the earth with 

 a force of about 15 pounds to the square inch. Mercury or quick- 

 silver, being 14 times heavier than water, the atmosphere will 

 support a column of this but about 29 or 30 inches in height; and 

 when a tube a little more than 30 inches long is closed at one end, 

 and filled with mercury, and then inverted into a basin also con- 

 taining mercury, the column will remain suspended nearly thirty 

 inches high. The mercurial column would continually remain at 

 the same elevation, if the atmosphere was subject to no variations; 

 but this is not the case; upon observation it is found subject to 

 continual variation, almost always falling before a wind arises, 

 and preceding rain, and again rising at the approach of calm 

 and fair weather. The instrument thus becomes a laromelcr, or 

 measurer of of the weight of the air. It is an invaluable instru- 

 ment on ship-board, giving indications of the coming tempest 

 long before any change is detected in the appearance of the sky. 

 Since the barometric column is wholly supported by the press- 

 ure of the atmosphere, communicated through the mobile parti- 

 cles of the fluid metal, to the open mouth of the tube, it obvious- 

 ly points out a method of determining heights. This however, is 

 from several causes, a matter of some nicety ; thus the density 

 of the air decreases as we rise upward, owing to the extreme 

 elasticity of air. It is well known that a piston may be thrust 



