110 EXPEHIMEyTS WITH PLANTS 



everywhere during rains. To study it more carefully, 

 we may construct a trough about twelve feet long, a 

 foot wide at the bottom, with sides six inches high. 

 This is best made in three sections, each four feet 

 long, fastened together end to end by strong hinges, so 

 that they may be inclined at different angles. The 

 lowest section should be level, the next iiiclined at an 

 angle of fifteen or twenty degrees, the next at an angle 

 of twenty -five to thirty -five degrees. Line the trough 

 throughout its entire length with a piece of oilcloth 

 twelve feet long and two feet wide: at the lower end 

 gather up the oilcloth and tie it firmly to a piece of 

 hose, which will serve to carrj^ off the waste water. 

 Leave the lowest section empty; fill the next section 

 with sand; fill the highest section with clay (placed in 

 the trough in a dry, powdered condition) or a mixture 

 of clay and sand (the clay should be pulverized and 

 mixed with the sand while dry) . Attach a piece of 

 rubber tubing to the faucet and allow a small stream 

 of water to trickle down the entire length of the 

 trough. The clay (or mixture) in the upper part of 

 the trough will behave in the same way as rock (only 

 the action will ])e much more rapid), and will show" 

 clearly how rock is sculptured by running water; how 

 masses of it become detached and fall off, and how as 

 these are carried down stream they lose their sharp 

 edges (this occurs especially where the clay overlaps 

 the sand). If different layers of clay (or mixture) are 



