44 



THE PRINCIPLES OF AGRICULTI'PE. 



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

 the column of mercury in the tube is heavy cnoug-h to 

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

 the more it will press, and the liigher the colunni 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- 

 })here 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- 

 mosi)here contains. It con- 

 sists of a tube, with a l)ulb 

 at the lower end, containing 

 mercury or alcohol. On tlie 

 principle that lieat expands 

 ami cold cduIimcIs, when the atmospher(^ becomes warm 

 the mercury or alcohol expands and rises in the tube, 

 and as (he atniosi)here becomes cooler it conlracls and 

 falls. 



Winds. — Since the atm()S]>here 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. 



