DEMONSTRATIONS IN PHYSICS 



221 



PIG. 107 



raising a disc attached to a balance from the surface of water, 



a small tank and the disc being brought into the field ; or 



a pencil of wood may be 



shown suspended by the 



cohesion of intervening 



fluid from the lower surface 



of another bar. 



114. Centrifugal Force. 

 Such an apparatus as shown 

 in fig. 107, or any other of 

 the usual forms, is very 

 easily projected, the rotating 

 portion only being brought 

 into the field, and the 



whirling-table supported as convenient. See also Plateau's 

 method as on p. 226. 



115. Hydrostatics. For 

 these projections, one or more 

 square or cubical glass tanks 

 should be provided. Using 

 such a tank about 5 in. cube, 

 the upward pressure of water 

 is easily shown in the usual 

 way by a smooth lead plate 

 supported against the bottom 

 of a glass tube. The princi- 

 ple of Archimedes and any of 

 its applications are projected 

 in a similar tank. The ap- 

 plication of these principles 

 to specific gravity, hydro- 

 meters, &c., may be shown by FlG 108 

 immersing three small test- 

 tubes, A, B, c (fig. 108), each loaded with the same small 

 weight of mercury or shot, either in larger tubes, or in three 



