10 More about the day 



better it acts. Enormous quantities of clay are used 

 for making bricks. Make some model bricks about an 

 inch long and half an inch in width and depth, also 

 make a small basin of about the same size, then set 

 them aside for a week in a warm, dry place. They still 

 keep their shape; even if a crack has appeared the 

 pieces stick together and do not crumble to a powder. 



If you now measure with a ruler any of the bricks 

 that have not cracked, you will find that they have 

 shrunk a little and are no longer quite an inch long. 

 This fact is well known to brickmakers ; the moulds in 

 which they make the bricks are larger than the brick is 

 wanted to be. But what would happen if instead of a 

 piece of clay one inch long you had a whole field of clay? 

 Would that shrink also, and, if so, what would the field 

 look like ? We can answer this question in two ways ; 

 we may make a model of a field and let it dry, and we can 

 pay a visit to a clay meadow after some hot, dry weather 

 in summer. The model can be made by kneading clay 

 up under water and then rolling it out on some card- 

 board or wood as if it were a piece of pastry. Cut it 

 into a square and draw lines on the cardboard right at 

 the edges of the clay. Then put it into a dry warm 

 place and leave for some days. Fig. 4 is a picture of 

 such a model after a week's drying. The clay has 

 shrunk away from the marks, but it has also shrunk all 

 over and has cracked. If you get an opportunity of 

 walking over a clay field during a dry summer, you 

 will find similar but much larger cracks, some of which 

 may be two or three inches wide, or even more. Some- 

 times the cracking is so bad that the roots of plants or of 

 trees are torn by it, and even buildings, in some instances, 

 have suffered through their foundations shrinking away. 



