THE WORK OF LEAVES 189 



to make one successfully in a few minutes. When 

 these arrangements are complete, apply vaseline to the 

 edges of the rubber disk and mouth of the chimney 

 (this precaution is important). Pour some water 

 into the chimney and force the piston slowly down 

 until the water stands half an inch or so above the 

 piston; the rubber bends back as it is forced down, 

 allowing the air to escape; when the piston is drawn 

 upward the wooden disk prevents it from bending and 

 so keeps the air from entering. Pull the piston back 

 nearly to the top of the chimney and secure it by 

 means of a clothes-pin, as shown in the figure; a lump 

 of sealing-wax fastened to the wire above the clothes- 

 pin prevents it from slipping back. Any leaks in the 

 piston may now be detected by inverting the chimney; 

 they may be stopped with sealing-wax if they occur in 

 the joints; if the leak occurs around the edge of the 

 rubber disk a larger one must be substituted. On the 

 other hand, the trouble may result from having too 

 large a rubber disk, in which case it will not lie flat 

 and a smaller one must be substituted. In withdraw- 

 ing the piston from the chimney, it is advisable to 

 draw it out slowly, with a twisting motion. 



A somewhat more convenient form of piston may 

 be made by a mechanic, after the plan shown in Fig. 

 108. It consists of a brass rod (about one-fourth of an 

 inch thick), with a thread at the end carrying two 

 nuts (Fig. 109, >^,w), a small washer (^^), and a brass 



