84 



Domestic Science 



cork, and string. Place the disc on one end of a cylin- 

 drical Argand lamp chimney with the string inside the 

 chimney, and hold the disc 

 firmly against the end of 

 the chimney by means of 

 the string. Lower the 

 chimney, disc end down- 

 wards, into a deep vessel 

 of water, as shown in 

 Fig. 25. When the disc is 

 some inches below the sur- 

 face of the water, release 

 the string. It will be found 

 that the disc does not fall 

 from the end of the chimney. 

 (If it does, the end of the 

 chimney does not fit the 



Fig. 25. 



upper surface of the disc tightly enough to form a 

 watertight seal. In this case, the end of the chimney 

 requires grinding to a flat surface by rubbing it, with 

 the chimney in a vertical position, upon the ground 

 glass surface of one of the plates used to cover gas -jars, 

 a little fine emery powder mixed with oil being used 

 as a grinding medium.) 



Evidently a force is acting upwards upon the lower 

 surface of the disc, pressing it against the end of the 

 chimney, and overcoming the force of gravity which 

 otherwise would cause the disc to fall. Carefully raise 

 the chimney, with the string still loose, and it will be 

 found that, at a certain point, the disc falls off, this 

 being evidently due to the force of gravity being now 

 slightly greater than the upward force. Note the 

 position of the lower end of the chimney when this 

 occurs, and measure the depth the end is then immersed 



