THE USE OF WEATHER INSTRUMENTS 



201 



readings of the thermometer are corrected before writing 

 them in the record. 



Exercise 50. Testing Thermometers 



Test your school and home thermometers for the freezing point, J 

 and for the boiling point if convenient (see Arts. 20 and 22). Your 

 teacher will advise you in the testing. Take the most accurate ther- 

 mometer found by the class 

 as a standard. Make cor- 

 rection cards for the others 

 like that in Fig. 139. 



227. The Need of Shel- 

 tering Thermometers. 

 In recording weather con- 

 ditions, thermometers 

 must have a proper ex- 

 posure. If three ther- 

 mometers are hung out 

 of doors in similar sur- 

 roundings except that one 

 is in the sun, another in 

 the shade over a grassy 

 plat, and the other in the 

 shade over pavements or 

 near buildings, there will 

 often be considerable dif- 

 ference in their readings. 

 Since the general weather 



. FIG. 140. Thermometer shelter; U. S. 



conditions are the Same Weather Bureau Pattern. Maximum and 



for all three thermometers, minimum thermometers are shown in posi- 



. tion The ordinary rain-gauge is seen at 



their different readings are the right. 



due to other causes. It 



may be the sun's rays shining directly upon the mercury; or it 



may be the heat reflected from pavements or buildings, etc., 



near by. To show correctly the temperature of a locality the 



thermometer must record the temperature of the free moving air 



and be shielded from other special effects. For this reason ther- 



