THE USE OF WEATHER INSTRUMENTS 199 



the bulbvjend is raised till the tube is nearly vertical. Then 

 the index slides down to the "top" end of the alcohol column. 

 Sometimes a slight jarring is needed to start the index. A 

 minimum after setting should read practically the same as an 

 accurate common thermometer beside it. 



224. Recording Thermometers. The Thermograph. The 

 thermograph writes a continous record of temperature. A 

 brass cylinder (Fig. 138) holds a recording sheet that is ruled 

 with vertical hour Jines and horizontal temperature lines. A 

 clock inside the cylinder turns it slowly, moving the record 

 sheet past the pen. The thermometer bulb is a flattened 



Self-Recording 



Thermometer. 



FIG. 138. Thermograph. The clock turns the cylinder round once each 

 week. The days and hours are marked by vertical lines; the degrees by 

 horizontal lines. The pen rises and falls with all changes in temperature. 



brass tube bent into a curve and filled with alcohol. One 

 end of this bulb is fastenedg'igidly to the frame, the other con- 

 nects with a set of^vers eliding at the pen. When the tem- 

 perature rises the pressure of the expanding alcohol gradually 

 straightens the curved tube and this raises the pen higher on 

 the recording sheet. Falling temperature contracts the alco- 

 hol and curves the bulb more; this moves the pen downward. 

 In this way the pen writes a complete record of the tem- 

 perature, showing all changes and the time when they occurred. 

 Thermographs are used at all W r eather Bureau stations, and 

 are in many high school laboratories. 



225. Obtaining Accurate Temperature Records. In mak- 

 ing a correct record of the temperature it is necessary: (1) 



