Let us take another step in order to emphasize more clearly the distribution of temperature over the United States on the first day of our series. Draw a line which shall separate all places having a temperature _above_ 30° from those having temperatures _below_ 30°, 30° being nearly the freezing point and, therefore, a critical temperature. Evidently this will help us to make our description of the temperature distribution more detailed. If this line is to separate places having temperatures _above_ 30° from those having temperatures _below_ 30°, it must evidently pass through all places whose temperature is exactly 30°. Examine the thermometer readings entered on your map to see whether there are any which indicate exactly 30°. You will find this reading at Norfolk, Va., Wilmington, S. C., Atlanta, Ga., Chattanooga, Tenn., Ft. Smith, Ark., and Portland, Ore. Through all these stations the line of 30° must be drawn. Begin the line on the Atlantic Coast at Norfolk, Va., and draw it wherever you find a thermometer reading of 30°. It is best to trace the line faintly with pencil at first, so that any mistakes can be easily rectified, and it should be drawn in smooth curves, not in angles. From Norfolk the line must run southwest through Wilmington, and then westward through Atlanta, passing just north of Augusta, which has 31°. From Atlanta the line goes northwest through Chattanooga, and thence westward, curving south of Memphis (28°) and Little Rock (26°), and then northwestward again through Ft. Smith.