198 THE WEATHER 



perature rises the mercury is forced through the narrowed 

 portion (B, Fig. 137) and may be seen slipping past in small 

 drops. When the temperature falls and the mercury in the 

 bulb contracts, the mercury in the tube remains there. Then 

 if the bulb end be held a little lower than the top, so that the 

 mercury is all joined together against the bulb end without 

 crowding any back into the bulb, the top of the column shows 

 the highest temperature reached since the instrument was last 

 "set." To set the maximum, lower its bulb end to a vertical 

 position. The mercury will then run past the narrowed point 

 until the bulb is full. Sometimes the maximum must be 

 jarred slightly, or whirled on its pivot, to make the mercury 



FIG. 137. Maximum and minimum thermometers. A marks the index in 

 the minimum and B the break in the mercury column of the maximum. 



run down. After setting, the maximum should read prac- 

 tically the same as an accurate common thermometer placed 

 beside it. The clinical or "fever" thermometer used by 

 physicians is a small maximum. 



223. Self-registering Thermometers. The Minimum. 

 The minimum thermometer contains alcohol and rests in a 

 horizontal position. Within the alcohol in the tube is a small, 

 double-headed, pin-like index (A, Fig. 137). Like other 

 liquids, alcohol has a film over its free surface. When the 

 temperature falls and the alcohol contracts, this surface film 

 draws the index back toward the bulb. When the temperature 

 rises, the index remains stationary and the expanding alcohol 

 runs past it. The upper end of the index, farthest from the 

 bulb, marks the lowest temperature reached since the in- 

 strument was last set. To set the minimum thermometer, 



