102 FURTHER ILLUSTRATIONS OF PRINCIPLES. 



The roller may be made of strong drawing-paper. A strip, 5 

 to 10 cm broad, and about O m '5 long, is wound round a test-tube 

 and the corners fastened with gum. A second strip, about half 



FIG. 72 (i real size). 



o 



FIG. 71 (real size}. 



FIG. 73 (| real size). 



as broad, is pasted upon the first and forms the thicker portion i 

 of the roller. According to the thickness of the paper used the 



