CH. Vl] TEMPERATURE. 159 



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

 supports the fulcrum of the writing lever must also be 

 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 

 to the upper end of the plant must be regulated. As it 

 is difficult to tie a knot at exactly the desired place, the 

 following plan, fig. 29, should be followed. Let an 

 assistant hold the lever up at the desired angle ; pass the 

 string or wire from the plant over the wire loop hanging 

 from the short end of the lever : pull it tight and secure it 

 by a folded piece of paper smeared on the inside with very 

 dense and quickly drying shellac varnish ; there will be 

 time to regulate the length before the varnish dries, and 

 when it is dry it is fairly secure. To make it still more 

 secure one or more ligatures of fine silk may be added 

 above the paper, but this is hardly necessary. If after all 

 the index is not at the right height, the fulcrum must be 

 slightly shifted up or down the supporting rod. The cord 

 from the plant to the lever must be vertical, which can be 

 insured by shifting the position of the flower-pot. 



Take a Narcissus growing in a flower-pot ^ and having 

 a scape in active growth, — the flower being still a young 



1 Narcissus will however grow quite well from a bulb dug up and kept 

 damp. 



