THE WEATHER 153 



On the daily weather maps we see curved lines (some- 

 times circles) called isobars. These lines connect points 

 having the same air pressure or " barometric pressure/ 7 

 Low pressure is associated with rain or wind, and high 

 pressure means fair weather. To measure it we use a 

 glass tube three feet long. This we first fill with quick- 

 silver, then close it with the thumb and set it up, with 

 its open end downward, in a cup full of quicksilver. 

 The quicksilver in the tube then falls down from the 

 closed end of the tube until it stops at about thirty 

 inches above the quicksilver in the cup. When a storm 

 is coming the quicksilver in the tube falls, sometimes as 

 low as twenty-eight inches; in fair weather it usually 

 stands at thirty inches or over, sometimes as high as 

 thirty and one half inches. When it falls fast, a heavy 

 storm is coming; when slowly, usually rainy weather; 

 after which it rises again. So when we know how the 

 barometer stands west of us, we know what kind of 

 weather is coming for many hours beforehand. 



We live like fish on the bottom of the air-sea, which is 

 at least forty miles * high, and probably a great deal 

 more. When we push down a cork in water it rises 

 as soon as we let it go, because it is lighter than water. 



1 Recent observations by the Belgian Government with sounding 

 balloons showed an air pressure of 0.67 inch at a height of about 18 

 miles. The highest sounding balloon ascension in North America 

 occurred on May 7, 1908, from St. Louis, under the direction of Pro- 

 fessor Rotch, when the height of 11 miles was reached. 



