THE CARTESIAN DIVER. 267 



upon the surface of water in a tall cylindrical vessel B, 

 over the mouth of which first a soft piece of bladder 

 md then a piece of cloth or other fabric is tightly 

 stretched arid tied ; the mouth of the cylinder 

 should for this purpose have a projecting rim. 

 If now strong pressure is applied by the hand to the 

 cover, the pressure is transmitted through the water 

 o the air enclosed in the bulb; the volume of the air 

 consequently diminishes,, and a small quantity of water 

 >enetrates into the diver, which becomes thereby 

 leavier and sinks. When the pressure is relieved, the 

 ur in the bulb expands, expels the excess of water 

 tfhich had entered it, and the diver Being now lighter, 

 'ises to the surface. Usually the ' diver ' is a small 

 igure of glass or porcelain, either attached to a bulb, 

 >r being hollow, and partially filled with air and water, 

 icts precisely upon the same principle as the bulb 

 done. 



The bulb may be filled by heating it over the spirit-lamp and 

 lipping the aperture in water ; the glass being very thin it bears 

 apid cooling without cracking. The air within, when heated, 

 'xpands and a portion escapes ; the remainder contracts when 

 gain cooled in the water and the latter enters to replace the air 

 v r hich has escaped. If a sufficient quantity of water did not enter 

 jhe bulb after the first heating, it is again held over the spirit-lamp, 

 lie heat now being applied to the space above the water already in 

 he bulb ; otherwise the water is heated and escapes, but not the 

 ir ; again, if too much water is in the bulb, a portion may be re- 

 loved by sucking or by heating it. 



When the diver is at the bottom of the cylinder, the 

 nclosed air bears not only the pressure of the hand 

 pplied to the cover, but also that of the column of 

 quid which is above it; hence when the pressure of 

 he hand is removed while the diver is at the bottom 



