SOME COMMON MACHINES 531 



Watch the effect upon the water in the pail. The shape which 

 the water assumes will be more evident if numerous small fragments 

 of paper or some saw dust be placed in the water. 



2. Place some water in a small pail and then swing the pail rapidly 

 in a vertical circle over your head. Can you do so without allowing 

 any of the water to fall out of the pail. Does this show that the 

 centrifugal force may be greater than the force of gravity? Explain. 



623. The Cream Separator. In a cream separator (Fig. 

 331) the heavier portions of whole milk, which constitute the 

 skim milk, are separated from the 

 lighter portions, which constitute 

 the cream, by centrifugal force. In 

 the type here illustrated, the whole 

 milk is fed slowly down through 

 the top opening 1 (Fig. 332). This 

 tubular shaft is closed at its lower 

 end but it has vertical slots in its 

 lateral wings, 2 and 2. Through 

 these vertical slots the milk passes 

 in thin sheets between rapidly re- 

 volving disks, 4 and 4, each of which 

 is shaped somewhat like an inverted 

 funnel. In the illustration the 

 space between the upper disks is 

 greatly exaggerated for the sake of 



,. . FIG. 331. The cream separator. 



clearness. These disks revolve at 



a rate of from 5000 to 15,000 r.p.m. (revolutions per minute). 

 Figure 333 shows the gearing by means of which this high 

 rate motion is obtained. 



The resulting centrifugal force is very great. The heavier, 

 i.e., denser, portion of the milk in each of the thin sheets is 

 thrown outward or against the under side of the disk above, 

 while the cream being lighter, i.e., less dense, remains against 

 the upper side of the disk just below. Thus it is that the 

 cream is separated from the skim milk. The cream accumu- 

 lates at the center of the separator as shown by the lighter 

 shading, 3, 3, in the illustration (Fig. 332), while the heavier 



