NATURAL HISTORY NINETEENTH CENTURY. 653 



area of comparative calm, which area has the lowest barometer reading, 

 while, receding from this, the readings gradually indicate higher and 

 higher pressures, until the limits of the storm are reached. It was in 

 1860 that the late Admiral Fitzroy, aided by the electric telegraph, 

 first established a system of warnings of storms and inaugurated public 

 weather forecasts in this country. In 1863 Mr. Galton published a 

 work entitled " Meteorographica," in which for the first time we have 

 charts of the weather, and it is to this author that we are indebted for 

 the general features. of the weather maps which the daily newspapers 

 have made familiar to all. Mr. Galton pointed out that areas of high 

 and of low barometric pressure are of vast extent, and that they pre- 

 sent certain regular features. Their forms, however, are constantly 

 changing, and the direction and rate of their movement vary greatly. 

 The law which governs cyclones and the direction of the wind is very 

 clearly laid down in " Meteorographica." We may first explain the 

 meaning of some terms. If the barometer readings, taken at any one 

 instant at a number of stations distributed over a large area of country, 

 be compared, certain series of stations will be found where the readings 

 have the same value, and if lines be drawn on maps through stations of 

 equal pressures/these lines, which are called isobars, will be found to ex- 

 hibit in general a certain parallelism, and not infrequently to form closed 

 curves. This will readily be understood by a glance on a weather 

 chart. Where a series of closed isobaric curves have values decreasing 

 inwards, the middle point of the system is spoken of as the centre of 

 a depression. If a line be drawn at right angles to a system of isobars, 

 the difference of the barometric readings for two stations on this line, 

 divided the distance between the stations, measures the gradient, which 

 is directly as the difference, and inversely as the distance. 



It might be supposed that difference of pressure is the cause of 

 winds, and that the air would pass directly from the areas of high 

 towards the areas of low barometric pressure ; that is, that the direc- 

 tion of the wind would generally be at right angles to the isobars. So 

 far, however, is this from being the case, that the opposite is the truth. 

 " One universal fact is," says Mr. Galton, " that on a line being drawn 

 from the locus of highest to the locus ot lowest barometer, it will in- 

 variably be cut more or less at right angles by the wind, and the wind 

 will be found to strike the left side of the line as drawn from the locus 

 of highest barometer. In short, as by the ordinary well-known theory, 

 the wind (in our hemisphere), when indraughted to an area of light- 

 ascending currents, whirls round in a direction contrary to the move- 

 ment of the hands of a watch; so conversely, when the wind disperses 

 itself from a central area of dense descending currents, or of heaped-up 

 atmosphere, it whirls round in the same direction as the hands of a 

 watch." 



Meteorologists have since reduced the general law of winds over all 

 the world to this very concise form : The wind blows along the isobars. 



