13 



and the cuiTcnt is thrown into a whirL A whirl may form and 

 rotate in the direction contrary to the hands of a watch ; by 

 centi-ifugal motion the air is thrown outwards, and there is an area 

 of low pressure within. This is called a cyclone. But sometimes 

 the whirl rotates in the opposite direction, and it has in this case a 

 centre of high pressure. This is known as an anticyclone. Each 

 of these forms of whirl gives rise to types of weather peculiar to 

 itself. 



The Barometer is the instrument with which the elastic 

 pressure of the atmosphere is measui-ed. As a weather-glass it fails, 

 the wording about it is nonsense : and, as a mere indicator of the 

 weather, a bit of dry sea-weed, pr the little toy cottage ^Tith the man 

 and woman in it, is better. The real use of the barometer, which 

 is very important, is to enable us to compare the pressure of the air 

 at different places. In the two types of whirls (the low-pressure 

 cyclone and the high-pressure anticyclone) the wind moves at right 

 angles to the direction of the gradient (to be mentioned presently), 

 and a hne of equal barometrical pressure can be drawn round the 

 central portion of the whiil. This is known as the line of equal 

 barometrical pressm'e, or isobar. Any number of isobars can be 

 drawn on the map, but a few only are sufficient. The isobars, taken 

 from actual obseiTation. are sketched three times a day for the 

 British Isles and North -West Eui'ope, and the lines are continued 

 conjecturally through the adjacent seas. Sometimes the log of a 

 passing ship corroborates the cui-ve. In both cyclone and anticyclone, 

 when the observer stands with his back to the wind, the barometer 

 is lower on his left hand than on his right. This rule is knoAvn as 

 Buys Ballot's Law. 



The difference of reading may be imagined to give a kind of 

 slope : thus, the barometer at one place at 30 inches and at another 

 at 29 inches, indicates an inchne. This is called a barometrical 

 gi-adient. The wind moves from one place to another across the 

 gradient, and the steeper it is the stronger is the wind. A difference 

 of 0*07 (or -jj-^) of an inch of barometrical pressiue in a space of 

 60 miles causes a rather strong wind ; when the gradient exceeds 

 this, a gale may be expected. But the force of the wind is not 

 regulated solely by the gi-adients ; there are other and obscure 



