MEASURING ATMOSPHERIC PRESSURE 



51 



Perier manufactured two tubes, filled them with mercury and 

 observed them in his garden at Clermont, the height of the mercury 

 in the tubes being 26 French inches and 3f lines. 



Leaving one behind to be observed during his absence, he took 

 the other up the Puy-de-D6me and, at the summit, 

 observed that the mercury had fallen in the tube to 

 23 inches and 2 lines. Noting the tube as he returned 

 to the lower levels of the mountain, he found that the 

 mercury continued to rise until by the time he 

 reached his starting-point the mercury stood at its 

 original level of 26 inches and 3f lines, at which point 

 that in the other tube had stood during his absence. 



Satisfied beyond measure with the result, Pascal 

 proposed this process as a means of determining the 

 height of any one place above another. Thus the 

 " barometer " was born, and sent on its career 

 throughout the civilized world. 



The most distinguished men of science have 

 worked to develop from this crude but original in- 

 strument of three hundred years ago the fine instru- 

 ment of the present day; yet the modern instrument 

 is nothing but the original " tube inverted in a 

 cup of mercury " with many refinements. 



Aneroid Barometer. The barometer is also used 

 to tell how high aeroplanes ascend, as the higher 

 they go the lower the barometer will drop. They 

 cannot, of course, use a mercury barometer. The 

 type of barometer used is called an aneroid barom- 

 eter (non-fluid). 



CONSTRUCTION 



FIG. 42. A 

 mercurial 

 barometer. 



A. Metal or base plate upon which parts are set. Fig. 44. 



B. Corrugated chamber of German silver (metal thickness 



0.0004 inch), from which all air is exhausted. It is secured to the plate A 

 by a screw which passes through the plate, and to which a nut is fastened. 



C. Bridge which spans vacuum chamber B. 



D. D. Adjusting screws which are used to either raise or lower the bridge, 

 thereby altering the tension on chamber B, 



