The word “High” is written inside the isobar marked 30.6, located in southern Oregon, and the same word is written inside the isobar marked 30.4, located on the South Atlantic coast, and also inside the isobar 30.04, which traverses Nova Scotia. These are the regions of great air pressure. The word “Low” is written at the center of the area inclosed by the isobar 29.6, which is situated in the State of Iowa. The latter is the region of least pressure. Sometimes there are several such regions shown on the weather map. =Why the Wind Blows.= Under the pull of gravity the atmosphere presses downward and outward, thus causing it to flow from the several regions of great pressure towards regions of less pressure. Observe the arrows, which fly with the wind, and it will be seen how generally this law is obeyed. The velocity with which the wind moves from the High toward the Low depends on differences in air pressure, modified in the lower stratum by the friction offered in passing over surfaces of varying degrees of roughness, the speed being greater over a water surface with the same difference in air pressure than over a level unwooded prairie, and greater over the open prairie than over an irregular wooded area. To illustrate: If the barometer were 30.5 at Bismark, Dakota, and 29.5 at Chicago, it would press upon the earth with a force of about seventy pounds greater per square foot at the first place than at the second. This difference in pressure would cause the air to flow from Bismark towards Chicago so rapidly that after allowing for the resistance due to friction on the earth there would remain a velocity of some fifty miles per hour, and Lake Michigan would experience a severe “Northwester”; and if the wind continued from the same direction for twenty-four hours a mighty sea would beat upon the eastern shore of the lake, and mariners and marine property would be at the mercy of a destructive tempest unless the Weather Bureau forecaster were alert and gave warning as soon as he saw such a juxtaposition of pressure distribution in the process of formation. We will give careful attention to this chart, for when its details are understood, others will be easily read. The chart shows a winter storm central in Iowa on December 15, 1893. The word “Low” marks the storm center. It is the one place in all the United States where the barometer reading is the lowest. The heavy black lines, oval and nearly concentric, about the Low, show the gradation of air pressure as it increases quite uniformly in all directions from the center of the storm outward.