8 SCIENCE IN SHOUT CHATTERS. 



The upright barometer, which shows the surface of the mer- 

 cury itself, is the most accurate instrument, provided it is care- 

 fully read. This form of instrument is always used in meteor- 

 ological observatories, where minute corrections 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 mer- 

 cury cistern. With such instruments, fitted with an appara- 

 tus called a " vernier," the height of the mercury may be read 

 to hundrcdths 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 portable barom- 

 eters 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 

 rilled with air ; one face of the box is corrugated, and so thin 

 that it can rise and fall like a stretched covering of india-rubber. 

 As the pressure of the outside air varies it does rise and fall, 

 and by a beautifully-delicate apparatus this rising and falling is 

 magnified and represented upon the dial. Such barometers are 

 made small enough to be carried in the pocket, and are very 

 useful for measuring the heights of mountains ; but they are 

 not quite so accurate as the mercurial barometer, and are 

 therefore not used for rigidly scientific measurements ; but 

 for all ordinary purposes they are accurate enough, provided 

 they are occasionally compared with a standard mercurial 

 barometer, and adjusted by means of the watch-key axis pro- 

 vided for that purpose, and seen on the back of the instrument. 

 They are sufficiently delicate to tell the traveller in a railway 

 whether he is ascending or descending an incline, and will in- 

 dicate the difference of height between the upper and lower 

 rooms of a three-story house. With due allowance for varia- 

 tions of level, the traveller may use them as weather indica- 

 tors ; especially as it is the direction in which the barometer 

 is moving (whether rising or falling) rather than its absolute 

 height that indicates changes of weather. Thus, by placing 

 the aneroid in his room on reaching his hotel at night, care- 

 fully marking its height then and there, and comparing this with 

 another observation made on the following morning, he may 

 use it as a weather-glass in spite of hill and dale. 



Water barometers have been made on the same principle aa 



