THE BAROMETER AXI) THE WEATHER. 183 



tub6, and watch it carefully from day to day, you will find that 

 the height of the column of mercury will continually vary. If 

 von 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 frequently 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 got he 

 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 difference ; and it 

 does so with such accuracy that by means of a barometer, or 

 rather of two barometers one at the foot of the mountain and 

 one on 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," because 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, and this cord 

 goes round a little grooved wheel to which the hands are 

 attached. Thus the rising and falling of the mercury moves 

 the float, the float-cord turns the wheel, and the wheel moves 

 the hands that point to the words and figures on the dial. 

 When this hand moves toward the right, or in the direction 

 of an advancing clock-hand, the barometer is rising ; when it 

 goes backward, or opposite to the clock-hand movement, the 

 mercury is falling. By opening the little door at the back of 

 such a barometer, the above-described mechanism is seen. In 

 doing this, or otherwise moving your barometer, be careful 

 always to keep it upright. 



It sometimes happens to these wheel barometers that they 

 suddenly cease to act ; and in most cases the owner of the 

 barometer may save the trouble and expense of sending it to 

 the optician by observing whether the cord has slipped from 

 the little wheel, and if so, simply replacing it in the groove 

 upon its edge. If, however, the mischief is caused by the 

 tube being broken, which is seen at once by the mercury having 

 run out, the case is serious, and demands professional aid. 



