156 SCIENCE IN SHORT CHAPTERS. 



isobar. But what, then, must be the course of the wind? 

 Will it be in straight lines towards this point? If so, a 

 strange conflict must result when all these currents meet 

 from opposite directions. What will follow from this 

 conflict? A skillful physicist can work out this problem 

 mathematically, but we are not all mathematicians, some 

 of us are not able to follow his formulas, 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. 



Tiiis is just what occurs when the air is rushing to fill a 

 comparatively 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 inwards, 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 illus- 

 trated. North of Dublin there is a curvature of isobars 

 and an inrush of winds towards 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 northwards as far as Dublin, and the high- 

 pressure focus has also moved northwards. 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 ou the 



