200 



THE WEATHER 



That the thermometers be accurate; (2) that the thermometers 

 be sheltered; (3) that a proper set of observation hours be 

 selected. 



226. The Need of Testing Thermometers. Not all ther- 

 mometers are reliable. Inaccuracies may come from several 

 causes. Two common defects are: (1) The scale may be 



CORRECTION CARD FOR MERCURIAL 

 THERMOMETER, No. 4875 



FIG. 139. 



placed too high, or too. low, on the tube, thus making all read- 

 ings too low or too high; (2) the bore of the tube may be 

 uneven, causing the mercury or alcohol to rise too slowly in 

 some portions and too rapidly in others. 



Every thermometer used by the Weather Bureau is care- 

 fully tested, and a correction card made showing all errors 

 (Fig. 139). Where errors are more than half a degree the 



