14 



Introduction to the Study of Science 



term mean? (See page 9.) A barometer for experimental 

 purposes should be made whether the school has a standard 

 barometer or not. 



Exercise : Making a barometer. You will need these materials : 

 a strong heavy glass tube with one end sealed, about thirty-six inches 

 long and with about a three-sixteenths inch bore ; a wide-mouth glass 

 bottle of four-ounce capacity ; a small quantity of clean mercury ; a 

 yardstick, and a wooden support for the tube and 

 yardstick, as shown in Fig. 4. The tube must be 

 clean ; to cleanse it use sulfuric ether, allowing it to 

 evaporate. In filling the. tube with mercury, hold 

 it at an angle of forty-five degrees and drop the 

 mercury slowly with a strong medicine dropper. 

 Remove all air bubbles either by means of a long, 

 straight wire to which is tied a small bunch of 

 thread, or by heating the glass until the air is 

 forced out. 



When filled, close securely with the finger, invert, 

 and place the end below the surface of the mercury 

 in the vessel, which should be about one third or 

 one half full. Remove the finger carefully so that 

 no air is admitted into the tube. A small piece of 

 wire gauze under the end of the tube will allow the 

 free passage of the mercury. The mercury at once 

 falls gradually to a certain point in the tube ; this 



FlG 4 A sim- indicates the pressure upon a corresponding surface 



pie barometer, show- of the mercury in the vessel. Fasten the tube 

 ing arrangement of securely to the support, and attach the yardstick 

 tube, yardstick, and ^^ the lower end exactly on a level with the 



surface of the mercury in the vessel. 



This simple barometer will meet all ordinary requirements, al- 

 though it is not as accurate as one needed for scientific observations. 

 You should compare it with a standard barometer, observing any 

 differences in the readings, and learn to correct possible errors. It will 

 give the best results if kept in a room where the temperature remains 

 fairly constant. Record readings daily at about the same hour. 

 Preserve the record for later use in making weather forecasts. 



It should be emphasized that a comparatively small difference in 

 barometric readings represents a considerable difference in the pres- 

 sure of the atmosphere. If 30 inches of mercury represent a pressure 

 of 14.7 pounds per square inch, what will 29 inches represent? Ex- 



