202 



THE SCREW-PROPELLER. 



Conversely, if a screw of this kind is fixed in such 

 manner that it may rotate but cannot move forward, it 

 will rotate if the water moves in the direction of th< 

 axis of the screw ; thus, if the water flows in the direc- 

 tion opposite to that of the arrow /, the screw wi] 

 rotate in the direction of the arrow d. 



FIG. 145 (an*proj.\ 



The glass cylinder of a lamp is presided with a ring, with 

 perforated strip across it, as for the experiment on the reaction 

 efflux ; the ring, however, is not fixed with sealing-wax, but pi 

 on loose so that it may be removed and put on again at will. Anothei 

 thin strip of metal, preferably of sheet brass, is cut 4 cm long and 5 r 

 broad, a cavity is made in the middle with the centre-punch, and 

 ends of the strip are bent, so that it may be pushed with modei 

 friction into the inside of the cylinder. The strip is fixed at aboi 

 the middle of the cylinder by placing a piece of sealing-wax aboi 

 the size of a pea at one end of the strip close to the glass and heat 

 ing the latter cautiously from outside until the wax melts ; 

 liquid wax will flow of itself into the space between the mei 

 and the glass. When one end is thus fixed and has become 

 cool, another small piece of wax is used in the same manner to fix 

 the opposite end of the strip. The lower end of the cylinder is 

 closed by a cork which has a hole about 8 or 10 mm wide in the 

 middle. The spindle of the screw is formed of a straight piece of 

 steel wire 3 mm thick, and somewhat longer than half the length of 



