FROM THE PRACTICAL VIEWPOINT 



which the milk flows at the top, and from which 

 it is discharged at the bottom. An inverted goose- 

 neck attached to the outlet pipe so directs the out- 

 flow that no milk can be discharged until the tank 

 has become filled. In theory, the hot milk which 

 enters at the top becomes gradually cooled as it 

 flows downward, and so the last milk to enter the 

 tank will be the last milk to leave it. If this 

 were entirely true, then the time required for fill- 

 ing a tank would be identical with the holding 

 time. In such case the various particles of milk 

 would pass through the tank like a flock of sheep, 

 one after another. Unfortunately, there are several 

 disturbing elements which enter into the problem, 

 and the particles of milk, instead of resembling 

 sheep, more nearly resemble a flock of chickens 

 in their behavior. In the first place, the milk 

 which is nearest to the outside circumference of 

 the tank becomes cooled more than that which is 

 in the centre, and as it cools it increases in weight, 

 and naturally falls to the bottom and goes out. 

 This tendency is partly overcome by providing in- 

 sulating jackets. In some cases, as in Fig. 25, 

 the jacket is a hot-water covering contained in 

 the space between the inner and the outer shell. 

 113 



