142 SCIENCE IN SHORT CHAPTERS. 



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 points to the words and 

 figures on the dial. When this hand moves towards the 

 right, or in the direction of an advancing clock-hand, the 

 barometer is rising; when it goes backwards, or opposite to 

 the clock-hand movement, the mercury is falling. By open- 

 ing 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 optican 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. 



The upright barometer, which shows the surface of the 

 mercury itself, is the most accurate instrument, provided 

 it is carefully read. This form of instrument is always 

 used in meteorological observatories, where minute correc- 

 tions are made for the expansion and contraction which 

 variations of temperature produce upon the length of the 

 mercury without altering its weight, and for the small 

 fluctuations in the level of the mercury cistern. With 

 such instruments, fitted with an apparatus called a "ver- 

 nier" the height of the mercury may be read to hundredth^ 

 of an inch. 



The necessity for the 30 inches of mercury renders the 

 mercurial barometer a rather cumbrous instrument: it must 

 be more than 30 inches long, and is liable to derangement 

 from the spilling of the mercury. On this account port- 

 able barometers of totally different construction have been 

 invented. The "aneroid" barometer is one of these the 

 only one that is practically used to any extent. It contains 

 a metal box partly filled with air; one face of the box is 



