16 Introduction to the Study of Science 



the air upon an equal area of the free surface of the water in the well. 



At normal atmospheric pressure this height is not more than thirty- 

 four feet; but it is usually somewhat less. If a 

 perfect vacuum were secured, how high would the 

 column of water rise? Does this agree with what 

 you have learned of the behavior of mercury in 

 the barometer? Draw a simple plan and explain 

 the operation of the suction pump. If the mercury 

 reading is thirty-one inches, how high would a water 

 barometer stand ? 



A different scale is sometimes used in our 

 country, although its use is mostly restricted 

 to laboratories for scientific research. This is 

 the metric scale. On this the average normal 

 atmospheric pressure reads 760 millimeters or 

 76 centimeters. A cubic centimeter of mercury 

 weighs 13.6 grams. What is the average air 

 pressure at sea level, expressed in grams per 

 square centimeter? 



13. Kinds of barometers in use. The 

 standard barometer such as that employed in 

 the stations of the United States Weather 

 Bureau is provided with a scale reading inches 

 and hundredths of an inch, and with an adjust- 

 able cistern of mercury (Fig. 5). Another 

 kind of barometer is the self-recording or baro- 

 graph (Fig. 6). This is a metallic barometer 

 equipped with a drum and operated by clock 

 FIG. 5. stand- WO rk. The record chart, which may be for 



ard barometer, of . 



the type used by a day or a week, is attached to the drum, 

 the United States The barograph is sometimes combined with a 



Weather Bureau. 



temperature recorder or thermograph. The 

 chart of such an instrument shows the pressure and the 

 temperature changes together with the hours and days of 

 the week. The barograph is constructed on the same principle 

 as the metallic or aneroid barometer. 



