140 CYCLONES. 
must become the column of air above me, therefore the column 
of quicksilver ought to gradually sink as I rise. Pascal actually 
performed this experiment, by climbing one of the Auvergne 
mountains with his tube of quicksilver, and he found that the 
quicksilver in the tube did sink lower and lower in exact pro- 
portion to the height he climbed. So in this way Torricelli’s 
theory, that the height of the column of quicksilver in the tube was 
an exact measure of the weight or pressure of the air outside, was 
proved to be true. We cannot now go up the side of a 
mountain to repeat Pascal’s experiment, but I have a simple 
experiment here which will perhaps do as well. Here is a tube 
of quicksilver similar to the one I made just now, with the 
column of quicksilver standing at the same height in it. But 
you will notice that the vessel of quicksilver into which the tube 
dips is underneath this bell-glass, which is placed above an air- 
pump. Now, if with this air-pump I remove some of the air 
from the bell-glass, then there will be less air pressing on the 
surface of the quicksilver, and we should expect the column of 
metal in the tube to sink, if it is true that it is kept up by the 
pressure or weight of the air. [Experiment made.] You see the 
column of quicksilver begins to sink as soon as the pump begins 
to remove the air from the bell-glass. This, then, is another way 
of proving that the column of quicksilver in the tube is kept up 
by the pressure of the air outside. 
The early experimenters with these tubes of quicksilver, or 
barometers, as we will now call them, for that is what they are, 
soon noticed that the height of the quicksilver standing in the 
tubes was not always constant in the same place, but that it 
varied, sometimes even to the extent of 23 inches; therefore, of 
course, it followed that the weight or pressure of the air in any one 
place was variable, When the column of quicksilver was noticed 
to sink much below the usual 30 inches—that is to say, when 
the air became much lighter than usual, bad weather often 
followed ; and when the column of quicksilver stood much above 
30 inches—that is to say, when the air became heavier than 
usual, fair weather was the rule. It was in ‘this way, then, that 
