88 THE FACTORS OF ORGANIC EVOLUTION. 



things around, there is an atmospheric pressure amounting 

 to about 15 pounds on the square inch : 15 pounds being 

 the average weight of a column of air having a square inch 

 for its base and extending upwards from the sea-level to 

 the limit of the Earth s atmosphere. He is made to observe 

 that when he puts one end of a tube into water and the 

 other end into his mouth, and then draws back his tongue, 

 so leaving a vacant space, two things happen. One is that 

 the pressure of air outside his cheeks, no longer balanced 

 by an equal pressure of air inside, thrusts his cheeks 

 inwards; and the other is that the pressure of air on 

 the surface of the water, no longer balanced by an equal 

 pressure of air within the tube and his mouth (into which 

 part of the air from the tube has gone) the water is forced 

 up the tube in consequence of the unequal pressure. Once 

 understanding thus the nature of the so-called suction, 

 he sees how it happens that when the plunger of the pump 

 is raised and relieves from atmospheric pressure the water 

 below it, the atmospheric pressure on the water in the well, 

 not being balanced by that on the water in the tube, forces 

 the water higher up the tube, so that it follows the plunger. 

 And now he sees why the water cannot be raised beyond 

 the theoretic limit of 32 feet: a limit made much lower 

 in practice by imperfections in the apparatus. For if, 

 simplifying the conception, he supposes the tube of the 

 pump to be a square inch in section, then the atmospheric 

 pressure of 15 pounds per square inch on the water in the 

 well, can raise the water in the tube to such height only 

 that the entire column of it weighs 15 pounds. Having 

 been thus enlightened about the pump s action, the action 

 of a barometer becomes intelligible. He perceives how, 

 under the conditions established, the weight of the column 

 of mercury balances that of an atmospheric column of 

 equal diameter ; and how, as the weight of the atmospheric 

 column varies, there is a corresponding variation in the 

 weight of the mercurial column, shown by change of 



