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.

[Illustration: Fig. 19.]

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.

In order to see what the connections between these ever-present, daily
phenomena are, and their connection with other phenomena, and that we may
understand their normal conditions, I will trace them approximately in a
diagram (figure 17.)

The foregoing diagram of the daily phenomena of a summer's day, when no
disturbing causes are in operation, no storm existing within influential
distance, and no unusual intensity or irregular action of any of the
forces present, affords a basis for considering the various phenomena of
the weather in all its changes and conditions.

It is obvious that the other phenomena do not all depend upon temperature
merely, if indeed any of them do.

Temperature has but one maximum and minimum, and that is exceedingly
regular, and does not correspond with any other.

The barometer has two; electric tension, two;