Electro-physiology and electro-therapeutics : showing the best methods for the medical uses of electricity / By Alfred C. Garratt.
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mean time, and that unceasingly for three years in succession, — during the nights as well as days, — shows that the tension of atmospheric electricity is at its minimum at 2 A. M. Setting out, then, from this hour, and we find the tension increases more rapidly, so that its value at 8 A. M. becomes almost double of that at 6 A. M.; but from this hour the increase becomes more gradual again until 10 A. M., the period of the first or morning maximum. Now, again setting out from this last-named hour, the tension declines gradually until 4 P. M., a period at which its value is only slightly superior to that of 8 A. M. This second minimum is called by the French philosopher Dc la Rive the diurnal minimum, to distinguish it from the nocturnal mini- mum, which takes place at 2 o'clock in the morning. After that hour (4 P. M.) the tension increases rapidly until 8 P. M., and after a slight further rise until 10 P. M., — the period of the principal or evening maximum, — when the ascending march of the electric tension is terminated. The evening maximum is very notably superior to that of the morning, or say at about 10 o'clock. Between 10 in the evening and midnight, the ten- sion of atmospheric electricity is decreased almost to the value of the diurnal minimum. It is occasionally observed on the same day that where the instruments mark 150° to 200° at the morning maximum, it runs down during the day so as to show only 3° to 4° at the afternoon minimum ; but it usually happens that variations so great are immediately followed by storms. We find that the air under a perfectly clear and serene sky, and when there is no storm within two or three hundred miles of us, is constantly, but moderately, positive ; but this is not always uniformly distributed through all the strata of the at- mosphere. It is found more nearly of the same intensity in a given horizontal stratum, but uniformly stronger in the upper strata, and stronger still in the ratio as we rise through the suc- cessive strata. But at the lowest stratum, i. e., at the surface of the ground where the air and earth unite, the electricity of the air is null. In the open country it does not commence to be sensible to most delicate instruments until about a yard or two above the ground. Where there are large trees, buildings, or