THE USE OF WEATHER INSTRUMENTS 187 



the frame through which we see the mercury column. The 

 vernier correctly shows the height of the column when its 

 lower edge is on a level with the highest point of the mercury 

 ("M,"Fig. 133). 



The reading of the barometer in the figure is. 28 in. 

 If 1 on the vernier were opposite the next higher tenth (28.1) 

 of the scale, the reading would be 28.01. Why? If 2 of the 

 vernier were opposite 28.2, the reading would be 28.02. Why? 

 If 4 of the vernier were opposite 28.4, the reading would be 

 28.04, etc. When this is clear, start with 28.1 of the scale and 

 move the vernier up 1 division at a time in the same manner. 

 For 28.11, 1 of the vernier is opposite the next higher tenth 

 of the scale, which is 28.2, and so on. Practice until clearly 

 understood. 



Using your scale and vernier, fill out the following: 



PROBLEMS 



1. When 2 on the vernier is opposite 28.2 on the scale the reading is.. . 28.02 



2. When 6 on the vernier is opposite 28.6 on the scale the reading is. . . ? 



3. When 1 on the vernier is opposite 28.2 on the scale the reading is.. . 28. 11 



4. When 3 on the vernier is opposite 28.4 on the scale the reading is. . . ? 



5. When 4 on the vernier is opposite 28.9 on the scale the reading is.. . 28.54 



6. When 1 on the vernier is opposite 29.8 on the scale the reading is. . . ? 



7. When 1 on the vernier is opposite 28.9 on the scale the reading is. . . ? 



8. When 9 on the vernier is opposite 29.1 on the scale the reading is. . . ? 



9. When 6 on the vernier is opposite 29.4 on the scale the reading is. . . ? 



10. When 7 on the vernier is opposite 30.0 on the scale the reading is.. . ? 



11. When 10 on the vernier is opposite 30.0 on the scale the reading is.. . ? 



12. When 9 on the vernier is opposite 29.9 on the scale the reading is.. . ? 



13. When 8 on the vernier is opposite 28.8 on the scale the reading is. . . ? 



The following rule may be followed in reading the barome- 

 ter to the Jloo m - by means of the vernier: Read off the 

 number of whole inches and the number of tenths of inches; 

 then running up the vernier, find the line which is just opposite 

 some line on the scale; the number of this line on the vernier 

 gives the number of hundredths of an inch. 



214. Correcting the Barometer Reading for Temperature. 

 Since mercury expands when heated (as in thermometers, 

 Art. 15), it follows that if two barometers, one warm and 



