26 



Some experiments with the scmd 



water on to the sand out of a watering can fitted with 

 the rose so as to imitate the rain. At first nothing seems 

 to happen, but if you look closely you will notice that the 

 water soaks through and does not lie on the surface ; it 

 runs right down to the clay ; then it comes out at the 

 tube there (c in the picture). None goes through the 

 clay, nor does enough stay in the sand to flow out through 

 either the top or the second tube ; of the four tubes only 

 one is discharging any water. The discharge does not 

 stop when the supply of water stops. The rain need only 

 fall at intervals, but the water will flow all the time. 



Fig. 13. 

 Model spring. A box with glass front contains a layer of 

 clay and one of sand. Water that falls on the sand 

 runs right down to the clay but can get no further, 

 and therefore flows out through the tube c at the 

 junction of the clay and the sand. The same result 

 is obtained when chalk takes the place of sand 



The experiment should now be tried with some chalk 

 from a quarry ; it gives the same results and shows that 

 chalk, like sand, allows water readily to pass. 



Just the same thing happens out of doors in a sandy 

 or chalky country ; the rain water soaks through the 

 sand or chalk until it comes to clay or solid rock that it 

 cannot pass, then it stops. If it can find a way out it 



