186 



THE WEATHER 



212. The Barometer. The common mercury barometer is a 

 Torricellian tube, mounted and provided with a scale for 

 reading the height of the mercury column. One type is 

 merely a barometer tube with its lower end bent upward 

 forming a sort of cistern to hold the mercury (Fig. 130). 

 Such an instrument is called a SIPHON barometer, and the 

 cheaper barometers on the market are usually of this pattern. 

 For use by the United States Weather Bureau and for other 

 accurate scientific work, barometers of FORTIN'S PATTERN (Fig. 

 131) are employed. In these the tube is straight; the sides 

 of the cistern are metal and glass; the bottom is stout buckskin 

 (Fig. 132), and is supported by a metal plate which can be 

 raised or lowered by a screw. Extending down from the top 

 of this cistern is an ivory point, .P. The scale 

 near the top of the barometer is so adjusted 

 that the line "26 in.," for instance, is exactly 

 26 in. above the tip of this ivory point. 



To read the barometer, first adjust the screw, s, 

 at the bottom so that the surface of mercury in 

 the cistern just touches the ivory point. Then 

 read the height of mercury in the column by 

 means of the scale. 



213. The Vernier. In order to read barome- 

 ters more accurately most instruments are pro- 

 vided with a VERNIER. This enables one to read 

 easily to Ho of the smallest division marked on 

 the scale, or to Hoo in. and Ho 



Exercise 49. To Construct a Vernier and to 

 Learn Its Use 



FIG. 133. 



The vernier. Using a stiff sheet of paper and a metric rule, lay 

 off carefully a scale like A, Fig. 133, 2 or 3 cm. wide 

 and 23 or 24 cm. long. Make each small division exactly 1 cm. long. 

 Let these divisions stand for ^f in., and number them as shown in 

 the figure. On a separate sheet lay off the VERNIER SLIDE, "B." 

 Here make each division exactly % Q cm. long. 



Note how the vernier on a barometer slides up and down on 



