94 PHYSIOGRAPHY. [chap. 



which are called isobars.'^ An isobar is simply a line con- 

 necting all those places which have, at a given time, the 

 same barometric pressure. Thus the first isobaric line, 

 reckoning from the bottom of the chart, passes through the 

 south of Ireland and the south-west of England, and then 

 sweeps in a bold curve through France into the Bay of 

 Biscay. At all points along this course the barometer stood 

 at 30 'I inches, as indicated by the figures at each extremity 

 of the curve. The next isobar, passing across the north of 

 Ireland and England, is marked 30 inches, so that between 

 the two lines there is a difference of pressure equal to that 

 of one-tenth of an inch of mercury. Another isobar 

 stretches across Scotland, and indicates a pressure of 29*9 

 inches ; and the last curve on the chart is drawn through 

 the north of Scotland, where the mercury stood at 29-8 

 inches. The chart therefore shows at a glance the distri- 

 bution of atmospheric pressure over the area represented, 

 and from this we can learn a good deal about the character 

 of the winds. Between two successive isobars there is a 

 difference of pressure represented by one-tenth of an inch 

 of mercury, and the distance between these two lines gives 

 us the gradie>it. This term is familiar enough in engineer- 

 ing ; if a railroad, for example, rises one foot for every 

 100 feet of distance, the line is said to have a "gradient " of 

 I in 100. The gradient is therefore the engineer's expres- 

 sion for the slope of the ground ; in like manner it is the 

 meteorologist's expression for what has been called the slope 

 of the atmosphere. Only, in talking about meteorological 

 gradients, it must be borne in mind that the vertical 

 scale is measured in hundredths of an inch of barometric 

 pressure, while the horizontal scale is measured in miles of 

 distance, the unit being one degree, or 60 nautical miles. 

 ^ Isobar, from Xaos, isos, equal ; and fiipos, baros, weight. 



