14 



More about the clay 



with holes into a funnel, press some clay on to it and on 

 to the sides of the funnel (Fig. 7), and then pour on rain 

 water. The water does not run through. Pools of water 

 may lie like this on a clay field for a very long time 

 in winter before they disappear, as you will know very 

 well if you live in a clay country. So when a lake or a 

 reservoir is being made it sometimes happens that the 

 sides are lined with clay to keep the water in. 



Fig. 7. 



A thin layer of clay a entirely 



prevents the water running 



through 



If water cannot get through can air? This is very 

 easily discovered : plug a glass tube with clay and see 

 if you can draw or blow air through. You cannot. Clay 

 can be used like putty to stop up holes or cracks, and 

 so long as it keeps moist it will neither let air nor water 



