44 



THE PRINCIPLES OF AGRICULTURE. 



cury in the cup or bag forces this up into the tube until 

 the column of mercury in the tube is heavy enough to 

 balance the pressure of the air. The heavier the air, 

 the more it will press, and the higher the column will 



rise in the tube. On the 

 other hand, the lighter the 

 air, the lower the column 

 will fall. 



Changes of temperature 

 and movements in the atmos- 

 phere cause the weight and 

 pressure to vary. As this 

 variation is correctly deter- 

 mined by the barometer, this 

 instrument becomes of great 

 service in calculating proba- 

 ble changes of weather. 



Temperature. The ther- 

 mometer is used to deter- 

 mine the temperature, or the 

 amount of heat that the at- 

 mosphere contains. It con- 

 sists of a tube, with a bulb 

 at the lower end, containing 

 mercury or alcohol. On the 

 principle that heat expands 

 and cold contracts, when the atmosphere becomes warm 

 the mercury or alcohol expands and rises in the tubo, 

 and as the atmosphere becomes cooler it contracts 'and 

 falls. 



Winds. Since the atmosphere is invisible, we are 

 apt to forget that it exists, until reminded by the force 

 of the wind that it is a real substance. Wind is simply 



* 



A Barometer Tube. 



