126 FERGUSON'S LECTURES. 



LEOT. to F; and then the syphon will stop, though the leg- 

 ^^^ C F should reach to the bottom of the cup. For which 

 reason, the leg that hangs without the cup is always 

 made long enough to reach below the level of its bot- 

 tom ; as from d to JE : and then, when the syphon is 

 emptied of the air by suction at E, the water immedi- 

 ately follows, and by its continuity brings away the 

 whole from the cup ; just as pulling one end of a thread 

 will make the whole clue follow. 



If the perpendicular height of a syphon, from the 

 surface of the water to its bended top at B, be more 

 than thirty-three feet, it will draw no water, even though 

 the other leg were much longer, and the syphon quite 

 emptied ot air ; because the weight of a column of 

 water thirty-three feet high is equal to the weight of 

 as thick a column of air, reaching from the surface of 

 the earth to the top of the atmosphere ; so that there 

 will then be an equilibrium, and consequently, though 

 there would be weight enough of air upon the surface 

 C to make the water ascend in the leg C B almost to 

 the height B, if the syphon were emptied of air, yet 

 the weight would not be sufficient to force the water 

 over the bend ; and therefore, it could never be brought 

 over into the leg BAG. 



Tantalus's Let a hole be made quite through the bot- 

 tom of the cup A, and the longer leg of the 

 bended syphon D E B G be cemented into 

 the hole, so that the end D of the shorter 

 leg D E may almost touch the bottom of the 

 cup within. Then, if water be poured into 

 this CUD, it will rise in the shorter leg by its 

 upward pressure, driving out the air all the 

 way before it through the longer leg: and 

 when the cup is tilled above the bend of the 

 syphon at F, the pressure of the water in the cup will 

 iorce it over the bend of the syphon ; and it will <1 P _ 



