Temperature, in summer, commences its diurnal elevation about 4 A.M., also, and rises till about 2 P.M. From that time it falls with very little variation till 4 o’clock the next morning. It has but one maximum and one minimum in the twenty-four hours.
As the morning barometric maximum approaches, and the heat increases the magnetic activity, condensation in the trade appears, or induced condensation in the upper portion of the surface atmosphere, that portion near the earth is affected and attracted—and the “wind rises,” according to the locality, the season, and the activity of the condensation. The tendency to blow increases with the tendency to trade and cumulus condensation, and continues till toward night, when it gradually dies away, unless there be a storm approaching. As the heat increases, and stimulates magnetism into activity, the magnetic needle commences moving to the west, its regular diurnal variation, and continues to do so until about 2 P.M., when it commences returning to the east, and so continues to return until 10 P.M., when it moves west again until 2 A.M., and from thence to the east, till 8 A.M.
Similar variations also take place in the horizontal force, as evinced by the action of the magnetometer needle, and in the vertical force, as shown by the oscillations. So that it is evident that there are two maxima, and two minima of magnetic activity every day, shown by all the methods by which we measure magnetic action and force—more than double at the acme of northern summer transit over that of winter, and proceeding pari passu, with the other daily phenomena—evincing the same irregular action which the other phenomena evince. Still another phenomenon, which has a daily change, is electric tension, or the increase or decrease in the tension of the positive or true atmospheric electricity.
Fig. 19.
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The following table shows the mean two hourly tensions for three years, at Kew, viz.:
| Hours | 12 P.M. | 2 A.M. | 4 A.M. | 6 A.M. | 8 A.M. | 10 A.M. |
| Number of observations | 655 | 784 | 804 | 566 | 1,047 | 1,013 |
| Tension | 22.6 | 20.1 | 20.5 | 34.2 | 68.2 | 88.1 |
| Hours | 12 A.M. | 2 P.M. | 4 P.M. | 6 P.M. | 8 P.M. | 10 P.M. |
| Number of observations | 848 | 858 | 878 | 874 | 878 | 1,007 |
| Tension | 75.4 | 71.5 | 69.1 | 84.8 | 102.4 | 104 |
From this it will be seen that the tension of electricity is at a minimum at 4 A.M., also, that it rises till 10, falls till 4 P.M., but not as rapidly, rises till 10, falls again till 4 A.M., or the close of the meteorological day—having two maxima and minima, as have most of the phenomena thus far considered.