CHILDREN'S GARDENS 149 



carbon and several other materials. It wraps the 

 whole earth like a big blanket to keep it warm from 

 the great cold outside. It can be weighed and meas- 

 ured, and it presses in every direction about fifteen 

 pounds to each square inch. It is this pressure that 

 makes it possible to pump, or breathe, or swallow. 



One of the important uses of moving air, is to 

 carry away the moisture and the gases which the 

 plants throw off, that is, to keep the plants well ven- 

 tilated. If this ventilation is stopped, the result will 

 be much the same as it is to people in a badly ven- 

 tilated room. The plants will get droopy and tired 

 looking, and the air inside of the bottles will become 

 much too warm for them to grow well. 



To illustrate the air pressure, take a glass tumbler, 

 fill it full of water, and place a card over the top 

 of it in such a way that there is no air left in the 

 tumbler. Hold this card firmly against the tumbler 

 and carefully turn it upside down. The child then 

 will readily see that the pressure of the air upward 

 against the card is great enough to support the weight 

 of the water in the tumbler. This will illustrate the 

 pressure of the air. 



One of the ways the garden soil is ventilated, 

 which means changing the air in it, is by the wind 

 blowing over the surface. This takes away some of 

 the downward pressure of the air, and when the down- 

 ward pressure is removed, the air in the soil comes 

 up, and when the wind stops, the full pressure is re- 

 newed, and the air moves downward again. This 



