c Science 



of observation has a considerable effect upon the reading 

 obtained and the predictions which may serve with fair 

 accuracy for low-lying stations, may be entirely mis- 

 leading for others at greater -altitudes. 



62. In most of the instruments and experiments 

 described in this chapter, the pressure of the air has been 

 balanced against that due to a column of mercury. 

 A classical experiment, due to Robert Boyle, in which 

 pressures are measured by the same means, will give us 

 valuable information upon another point, namely, the 

 influence of pressure upon the volume of a gas. 



EXPERIMENT 35. Close one end of a piece of glass 

 tubing, about 50 inches long, and with a bore of at 

 least J". Bend the tube round on 

 itself at a distance of about 10" from 

 the closed end, so that its appearance 

 is as shown in Fig. 31. Clamp the 

 tube in an upright position. Insert 

 a small funnel into the upper end of 

 the long limb, and carefully pour in 

 mercury till the liquid closes the 

 bottom of both tubes and is at the 

 same level in each. The air in the 

 short limb is thus cut off from the 

 outside atmosphere. The pressures 

 on the free surfaces of the mercury 

 must be equal, since the mercury is 

 at the same level in both arms of 

 the tube, and is at rest. The pres- 

 sure on the mercury surface in the 

 long limb is evidently that of the 

 atmosphere. Measure this pressure 

 by means of a barometer. Measure 

 the length of the column of air contained in the 



Fig. 31. 



