TABLE IV.——REDUCTION OF BAROMETER TO SEA LEVEL. The figures in heavy type, in the left-hand vertical columns, are the heights, in feet, of the barometer above sea level. The figures in heavy type at the top of the columns, running across the page, are the readings of the ordinary thermometer. The numbers of inches and hundredths of inches to be subtracted from the barometer reading (corrected for temperature by Table III), for the different heights above sea level, are given in the vertical columns. The altitude above sea level of the city or town at which the observation is made should be ascertained as accurately as possible from some recognized authority, as, _e.g._, from a railroad survey; from Government measurements, or from some engineer’s office. The correction to be made is determined by a simple inspection of the table or by the method of interpolation. _Example_: Altitude of Barometer above sea level, 840 feet; Temperature of the air, 40°; Correction, +.931 inch. _Example_: Altitude of Barometer above sea level, 205 feet; Temperature of the air, 45°; Correction = ? Here 205 feet and 45° are neither of them found in the table. Hence a double interpolation is necessary. For 200 feet and 40° the correction is +.224 inch. For 200 feet and 50° the correction is +.220 inch. Hence for 200 feet and 45° the correction is +.222 inch. For 210 feet and 40° the correction is +.235 inch. For 210 feet and 50° the correction is +.231 inch. Hence for 210 feet and 45° the correction is +.233 inch. Now for 205 feet we should have a correction midway between +.235 inch and +.233 inch or +.234 inch. TABLE I.——TEMPERATURE OF THE DEW-POINT, IN DEGREES FAHRENHEIT.