THE BAROMETER AND THE WEATHER. 141 



mercury balances the pressure of the many miles of atmos- 

 phere, and is supported by it. Thus the column of mer- 

 cury may be used to counterbalance the atmosphere and 

 show us its weight; and such a column of mercury is a 

 barometer, or "weight measure." The word barometer is 

 compounded of the two Greek words laros, weight, and 

 metron, a measure. 



If you take a glass tube a yard long, stopped at one 

 end and open at the other, fill it with mercury, stop the 

 open end with your thumb, then invert the tube and just 

 dip the open end in a little cup of mercury, some of the 

 mercury in the tube will fall into the cup, but not all; only 

 six inches will fall, the other 30 inches will remain, with 

 an empty space between it and the stopped end of the tube. 

 When you have done this you will have made a rude bar- 

 ometer. If you prop up the tube, and watch it carefully 

 from day to day, you will find that the height of the column 

 of mercury will continually vary. If you live at the sea- 

 level, or thereabouts, it will sometimes rise more than 30 

 inches above the level of the mercury in the cup, and fre- 

 quently fall below that height. If you live on the top of a 

 high mountain, or on any high ground, it will never reach 

 30 inches, will still be variable, its average height less than 

 if you lived on lower ground; and the higher you go the 

 less will be this average height of the mercury. 



The reason of this is easily understood. When we ascend 

 a mountain we leave some portion of the atmosphere below 

 us, and of course less remains above; this smaller quantity 

 must have less weight and press the mercury less forcibly. 

 If the barometer tells the truth, it must show this differ- 

 ence; and it does so with such accuracy that by means of 

 a barometer, or rather of two barometerg one at the foot 

 of the mountain and one at its summit we may, by their 

 difference, measure the height of the mountain provided 

 we know the rules for making the requisite calculations. 



The old-fashioned barometer, with a large dial-face and 

 hands like a clock, is called the "wheel barometer," be- 

 cause the mercury, in rising and falling, moves a little glass 

 float resting upon the mercury of the open bent end of the 

 tube; to this float and its counterpoise a fine cord is attached; 



