THE BEGINNINGS OF EXPERIMENTATION 57 



been important in biology and astronomy. The 

 thermometer, also invented by Galileo, and the manom- 

 eter complete the list of the most important. The 

 last is an instrument for measuring the pressure of 

 gases and is a development of the apparatus used by 

 Boyle in his famous demonstration that ah- has elas- 

 ticity of volume. 



Boyle's experiments on the " spring of the air," 

 as he called it, may be illustrated by the apparatus of 

 Fig. 4. Some air is trapped in the short and 

 sealed end of the U-shaped tube by pouring 

 mercury into the long end. If the heights of 

 the mercury in the two arms of the tube are 

 the same, the air in the closed end is at atmos- 

 pheric pressure. The column of atmosphere 

 acting on the mercury of the open arm is 

 equivalent to the column of mercury of a 

 barometer. For convenience in describing the FlG 

 experiment let us assume that this is 30 inches. 

 If more mercury is poured into the long tube the levels 

 are altered. The pressure on the trapped air is greater 

 than atmospheric pressure by the amount which the 

 column of mercury in the open tube exceeds that in 

 the other. If this distance is 6 inches, the total pres- 

 sure on the inclosed air is that of 36 vertical inches 

 of mercury. The volume of this air is, of course, re- 

 duced. The new volume will be found to be f of the 

 original volume. In general, if the pressure is changed 

 the volume is changed inversely, a fact known as 

 Boyle's Law. 



We now see that such a device may be used as a 

 manometer by connecting its long arm to the container 



