On Water and Air. 
[August, 
512 
to the illustrious Robert Boyle. One experiment which has 
been made has an immortal interest, and is of the utmost 
importance in relation to this subject. I have here (Fig. 37) 
a simple instrument, similar to the one with which Boyle 
operated. The long arm (a) of the bent tube (a b) is open 
at the top. The short arm is closed at B. We have intro- 
duced some mercury into the bent part of the tube, and the 
fluid metal now stands at the same level (at o) in both the 
long and short arms. The space above the mercury in the 
closed tube contains a certain bulk of atmospheric air, sub- 
mitted to the ordinary pressure of the air through the open 
tube A. And now I will ask Mr. Cottrell to pour carefully 
into this longer arm a quantity of mercury. What will 
occur ? The mercury falling into the tube will press the 
mercury which is in the bent part up into the shorter arm. 
The air in this short arm will be squeezed more forcibly 
together, and the greater the amount of mercury which we 
Fig. 37. 
J3 
