MOTIONS AND PEESSUEE OF THE ATMOSPHEEB. Ill 



example it may be stated that during the great storm of January 26th, 

 1884, the barometer in Leith fell to 27 60 inches, and at Kilcreggan, near 

 Greenock, to 27'32 inches. 



At the latter end of this work we give some further remarks on the 

 Barometer, including those by the late Admiral FitzEoy, which may be 

 referred to in connection with what has been stated in the foregoing pages 

 on this subject. 



Having thus briefly recorded the chief points to be observed in making 

 use of the indications afforded by the Barometer, we now proceed to deal 

 with the second part of this Section, referring to the Motions and Pressure 

 of the Atmosphere. 



(28.) Mr. W. Clement Ley, in the pamphlet previously quoted on p. 104, 

 says : — Changes of Weather are closely related to changes of Wind, and 

 changes of Wind to changes in the distribution of Atmospheric Pressure. 

 It would not indeed be safe to assert that the weather conditions are, in 

 all cases and absolutely, the effects of atmospheric currents, or that the 

 atmospheric currents are absolutely the effects of the distribution of 

 pressure ; for the inter-action between these several elements is not quite 

 so simple as would be conveyed by such an assertion. When the atmo- 

 sphere is in a state of equilibrium no movement of air can of course occur, 

 but as soon as the equilibrium is disturbed a current of wind is generated, 

 the air tending to move from the district of greater to the district of Lss 

 pressure ; in other words, the heavier air tends to drive out the Ughter. 

 Absolute equilibrium is in nature hardly ever attained, the air being kept 

 in almost perpetual agitation by changes of pressure. The prime cause of 

 atmospheric disturbance is found in the unequal distribution of solar heat 

 over the earth's surface. 



As soon as a current of wind is established, the effect of the earth's 

 rotation on its axis is to cause this current to deviate to the right of what 

 would otherwise be its course in the Northern Hemisphere, and to the left 

 in the Southern. The winds thus blow to a certain extent round instead 

 of directly into any area in which the pressure of the atmosphere is rela- 

 tively low, and the centrifugal force of the currents themselves helps in 

 this case to maintain the inequality of pressure. 



This effect of the force of the earth's rotation varies with the latitude. 

 In the high latitudes we find that if we describe lines along which at any 

 given time the pressure of the atmosphere at the level of the sea is equal, 

 while it is higher on the one side and lower on the other, the wind blows 

 in a direction which, for practical purposes, may be regarded as parallel to 

 those lines. In low latitudes, on the other hand, the winds move more 

 nearly at right angles to such lines, and at the Equator the deflecting 

 effect of the earth's rotation is zero, and the air must consequently there 

 travel in direct lines from the points at which the pressure is greatest to 

 those at which it is least. 



(29.) The intimate connexion between the fluctuations of the Barometer 

 and changes in wind and weather is famihar to all. As Mr. Piddington 

 says : " The commander who is watching his Barometer is watching his 

 snip, and that in the most eflicient manner." This remark has a greater 



