14 



EXPERIMENTS WITH PLANTS 



needle substituted for it; the varnish is burned off or 

 scraped away. The upright piece is firmly wedged in 

 a hole bored in a small block of wood (see Fig. 17). 



The pans are made of tin covers, 

 attached with silk thread. Their 

 weights may be equalized by 

 trimming the edges or by attach- 

 ing a little sealing-wax to them; 

 when balanced they should hang 

 about an inch and a half above 

 the table. Another kind of sup- 

 port, made by fixing two short 

 glass tubes to a strip of wood (by 

 means of sealing-wax) is shown 

 in Fig. 18. 



When the balance is set up, test it carefully; first 

 get the pans to balance; then put weights in both 

 pans until they balance; then exchange the weights; 

 if the balance is properly made they should still 

 balance; failure to do so shows that the pans are not 

 of equal weight, or are not attached at equal distances 

 from the pivot. An inaccuracy in the balance will 

 not vitiate your results for comparative purposes if 

 you always put the weights in the same pan. Why? 

 If you cannot obtain weights, make some of lead 

 (to correspond with a druggist's; the metric system 

 is much more convenient than the ordinary apothe- 

 caries' weight). 



18. A modification of 

 the balance. 



