CH. VI] 



TEMPERATURE. 



145 



ratus the following points must be noted. Let the paper 

 be smooth and tight on the drum, to insure which the 

 unglazed side of the paper should be wetted with a 

 sponge before it is attached. Do not smoke the paper too 

 heavily : a paraffin stove-lamp, with the long wick turned 

 up so as to flare, may be used for the smoking. The 

 drum should be accurately vertical, which may be tested 

 by a spirit-level placed on the top. The stand which 

 supports the fulcrum of the writing lever must also be 



Fig. 27. Exp. 186. 

 vertical. Unless these conditions are fulfilled the index 

 will write at the top of its course, but will leave the 

 surface, or press too hard against it, lower down. The 

 fulcrum should be opposite the middle of the drum, i.e. 

 equally distant from the upper and lower edges of the 

 smoked paper. When the experiment begins the index 

 must touch the paper near its upper edge. To insure this 

 the length of the string joining the short arm of the lever 



D. A. 10 



