266 



THE CARTESIAN DIVER. 



the heating of one end, or when the piece is soldered on, it is ad- 

 visable to wrap a piece of wet rag round the portion .not to be 

 heated. The screw is made of a stout piece of brass wire, 4 mm 

 thick, which is soldered at one end into a knob of thick sheet brass 

 (made six- or eight-sided by filing) after the screw has been cut 

 upon it, or otherwise the wire is apt to be broken off the knob again 

 while the screw is cut. The little piece which forms the knob is 

 also tapped, the screw is left a little longer than required, 

 screwed into the knob, a small piece of solder is laid on carefully 

 to prevent its running down upon the threads of the screw, 

 and finally the portion of the screw which projects on one 

 side beyond the flat knob is nipped off, and filed smooth. A forward 

 motion of the screw opens the pinch-cock, which may thus be 

 adjusted to any required aperture. 



Fig. 185 A represents a small hollow bulb of glass, 

 open below, which contains air and water in such 



FIG. 185 (A real size, B \ real size}. 



quantities that it floats nearly immersed and that very 

 little mere water is required to make it sink. Such a 

 body is usually called a Cartesian diver ; it is placed 



