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The pressure of the atmosphere on the earth is equal to that 

 of an ocean of quicksilver thirty inches deep, and it is a fact 

 which from its familiarity does not excite the wonder due to it, 

 that this atmospheric ocean is liable to be disturbed by waves, 

 which, as the barometer shows, sometimes attain to a height of 

 at least one fifteenth of its depth. Regions of high barometer 

 are generally those of fine weather, and regions of low barometer 

 those of wet weather, because in the latter ascending currents 

 of air are formed, which are due to the pressure of the air in 

 the neighbouring regions of high barometer. These as they 

 ascend become cooled, and condense the watery vapour which 

 they contain into clouds and rain. At the equator, where the 

 rain-fall is very great, the fluctuations of the barometer are very 

 slight, and it would be the same in all parts of the world were 

 it not for the deflecting effect of the earth's rotation. The 

 simplest instance of this effect is that, as theory and observation 

 alike show, in the northern hemisphere a cannon ball fired at a 

 sufficiently distant mark strikes a point a little to the right of 

 the mark. In the southern hemisphere the corresponding de- 

 flection is to the left ; and at the equator, where the earth has 

 no rotation in relation to an axis vertical to the horizon, there 

 is no deflection. The deflection is caused by the earth moving 

 in its rotation under the cannon ball ; the cannon is in fact fired 

 at a moving mark ; — and in the same way, the earth rotating under 

 a current of wind deflects the wind in the northern hemisphere 

 to the right ; so that every north wind tends to become an east 

 wind, and every south wind tends to become a west wind. 

 In the southern hemisphere, this effect is of course reversed. 

 This effect of the earth's rotation on the winds was first pointed 

 out by Professor Dove, of Berlin, forty or fifty years ago, and 

 is called Dove's Law ; but Mr. Murphy said he believed Dr. 

 Hann had been the first to see the full importance of this law. 

 Theory and observation alike show that the fluctuations of the 

 barometer increase as the distance from the equator increases. 

 They are almost nothing at the equator. At a latitude of 65 

 the average monthly fluctuation is nearly an inch and a half. 



