wiXDS, 245 



water has all been drawn off. Difficulties, however, some- 

 times occur. If the tube is small and very long, and the 

 descent is trifling, the friction of the water in the tube 

 may prevent success. Water iisually gives out small 

 quantities of air, which collects in the higher part of the. 

 syphon, and after a while fills it, causing the stream to 

 cease running ; but syphons for this purpose, when only a 

 few rods in length, wdth several feet descent, are usually 

 found to succeed well. If tlie discharging orifice is sev" 

 eral times smaller than the tube, it is frequently of material 

 use, by causing a slow and steady current through the 

 syphon. 



CHAPTER II. 



MOTION OF AIR. 

 "WINDS. 



Wind is air in motion. Its force depends on its speed. 

 When its motion is slow, it constitutes the soft, gentle 

 breeze. As the velocity increases, the force becomes 

 greater, and the strong gale sweeps around the arms of 

 the wind-mill with the strength of many horses, and huge 

 ships are driven swiftly through the waves by its press- 

 ure. By a still greater velocity of the air, its power be- 

 comes more irresistible, and solid buildings totter, and 

 forest trees are torn up by the roots in the track of the 

 tornado. 



The force of wind increases directly as the square of 

 the velocity. Thus a wind blowing ten miles an hour ex- 

 erts a pressure four times as great as at five miles an hour, 

 and twenty-five times as great as at two miles an hour. 



