144 SCIENCE IN SHORT CHAPTERS. 



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 at- 

 mospheric fluctuation. 



But why should the height of the barometer vary white 

 it remains in the same place? 



If the quantity of air surrounding the earth remains ihe 

 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 MX- sre 

 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 ane 

 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 not only of the permanent gases, 

 nitrogen and oxygen; it contains besides these and carbonic 

 acid, a considerable quantity of gaseous matter, which is 

 not permanent, but which may be a gas at one moment 

 contributing its whole weight to that of the general atmos- 

 phere and at another moment some of it may be con- 



