THE BAROMETER AND THE WEATHER. 143 



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 

 net used for rigidly scientific measurements; but for all 

 ordinary purposes they are accurate enough, provided they 

 are occasionally compared with a standard mercurial bar- 

 ometer, and adjusted by means of a watch-key axis pro- 

 vided for that purpose, and seen on the back of the instru- 

 ment. They are sufficiently delicate to tell the traveller in 

 a railway whether he is ascending or descending an incline, 

 and will indicate the difference 01 height between the upper 

 ai>d lower rooms of a three-story house. With due allow- 

 ance for variations of level, the traveler may use them as 

 weather indicators; 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, carefully 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 

 as the mercury barometer; but as water is 13 times 

 lighter, bulk for bulk, than mercury, the height of the 

 column must be 13 times 30 inches, or, allowing for vari- 

 ations, not less than 34 feet. This, of course, is very 

 cumbrous; the evaporation of the water presents another 

 considerable difficulty,* still such a barometer is a very in- 

 teresting instrument, as it shows, the atmospheric fluctua- 

 tions on 13 times the scale of the ordinary barometer. 

 A range of about five feet is thus obtained; and not only 



* This has been recently overcome to a great extent by using gly- 

 cerine instead of water. 



