THE BAROMETER AND THE WEATHER. 185 



the mercury barometer ; but as water is 13-J- times lighter, 

 bulk for bulk, than mercury, the height of the column must be 

 13^ times 30 inches, or, allowing for variations, 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 interesting instrument, as it shows the 

 atmospheric fluctuations on 13^ times the scale of the ordinary 

 barometer. A range of about five feet is thus obtained ; and 

 not only the great waves, but even the comparatively small 

 ripples of the atmospheric ocean are displayed' by it. In 

 stormy weather it may be seen to rise and fall and pulsate like a 

 living creature, so sensitively does it respond to every atmos- 

 pheric fluctuation. 



But why should the height of the barometer vary while it 

 remains in the same place ? 



If the quantity of air surrounding the earth remains the 

 same, and if the barometer measures its weight correctly, why 

 should the barometer vary ? 



Does the atmosphere grow bigger and smaller, lighter and 

 heavier, from time to time ? 



These are fair questions, and they bring us at once to some 

 of the chief uses of the barometer. The atmosphere is a great 

 gaseous ocean surrounding the earth, and we are creeping about 

 on the bottom of this ocean. It has its tides and billows 

 and whirling eddies, but all these are vastly greater than those 

 of the watery ocean. At one time we are under the crest or 

 rounded portion of a mighty atmospheric wave, at another the 

 hollow between two such waves is over our heads, and thus the 

 depth of atmosphere, or quantity of air, above us is variable. 

 This variation is the combined result of many co-operating 

 causes. In the first place there are great atmospheric tides, 

 caused, like those of the sea, by the attraction of the sun and 

 moon ; but these do not directly affect the barometer, because 

 the attracting body supports whatever it lifts. Variations of 

 temperature also produce important fluctuations in the height 

 and density of the atmosphere, some of which are indicated by 

 the barometer others are not. Thus a mere expansion or 

 contraction of dry air, increasing the depth or the density of 

 the atmospheric ocean, would not affect the barometer, as 

 mere expansion and contraction only alter the bulk without 

 affecting the weight of the air. But our atmosphere consists 



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

 glycerine instead of water. 



