196 SCIENCE IK SHORT CHAPTERS. 



to find their level, and thereby the winds are produced. The 

 air flows from all sides toward the lowest isobar. But what, 

 then, must be the course of the wind ? Will it be in straight 

 lines toward this point ? If so, a strange conflict must result 

 when all these currents meet from opposite directions. What 

 will follow from this conflict ? A skilful physicist can work 

 out this problem mathematically, but we are not all rnathe- 

 hnaticians, some of us are not able to follow his formula), and, 

 'therefore, will do better by resorting to simple observation of 

 other analogous and familiar phenomena. A funnel or any 

 vessel with a hole in the bottom will answer our purpose. Let 

 us fill such a vessel with water, then open the hole and see 

 what will be the course of the water when it is struggling to 

 flow from all sides to the one point of vacuity. It will very 

 soon establish a vortex or whirlpool i.e. the water instead of 

 flowing directly by straight lines from the sides to the centre 

 of the funnel, will take a roundabout, spiral course, and thus 

 screw its way down the outlet of the funnel. 



This is just what occurs when the air is rushing to fill a com- 

 paratively vacuous atmospheric space. It moves in a spiral ; 

 and in the Northern Hemisphere this spiral always turns in the 

 same way viz. in the opposite direction to the hands of a 

 clock when flowing inward, and vice versa, or with the clock 

 hands, when the air is overflowing from a centre of high 

 pressure. 



In the chart for October 5th both these cases are illustrated. 

 North of Dublin there is a curvature of isobars and an inrush 

 of winds toward a northward low pressure, or vacuous region ; 

 while south of Dublin the isobar tends sharply round a high- 

 pressure focus, and the overflowing wind is correspondingly 

 reversed in direction, as shown by the arrows. 



The next chart, for October 6th, shows that the overflow has 

 spread northward as far as Dublin, and the high-pressure focus 

 lias also moved northward. It follows from this that if you 

 know the barometric gradient, and stand with your left hand 

 to the region of low barometer and your right hand to that 

 of the high barometer, the wind will blow against your back 

 i.e. you will face the direction of the wind, or of those 

 flying arrows on the chart. This interesting and important 

 generalization is called " Buys Ballot's Law." In spite of the 

 proverbial fickleness of the winds this simple law is rarely 

 infringed, though it may require a slight modification of 

 statement inasmuch as the wind does not move in circles round 



