1. In descending from the surface of the earth, to depths continually augmenting, the mean temperature of the year augments gradually; yet, immediately below the surface, and at depths of half a foot or a foot, the mean temperature is found to be a minimum.
2. The rate at which the annual variations of temperature are transmitted to the interior of the earth, may be estimated at 6 or 7 days for 1 foot thickness of earth.[1]
3. Observation and theory agree in showing that the extreme temperatures of the year decrease in geometrical progression, while the depths below the surface are taken in arithmetical progression.
4. The annual variations of temperature may be considered as insensible at depths from 60 or 75 feet; that is to say, at the depths where the maxima and minima will occur at the same epochs (after an interval of one year!) as at the surface.
5. On descending several feet below the surface, the annual variations of temperature are as the sines of the elapsed times, in a circle whose circumference corresponds to the period of one year.
6. When different latitudes are compared, it appears that the annual variations of temperature penetrate to the least depths in the higher latitudes.
7. The rate with which diurnal variations of temperature are transmitted to the interior of the earth, may be stated at somewhat less than 3 hours for 1 decimetre in thickness (3.9 inches English).
8. The diurnal variations become insensible at a depth of 1.3 metre (51 inches), which is 19 times less than the depth reached by the annual variations, as theory also indicates.
The important conclusion of the entire disappearance
- ↑ Forbes's experiments in different sorts of rock show the effect of these, in modifying the range of subterranean temperature, in altering the rate of its progress, and changing the epochs of maximum and minimum temperature. (Edinb. Trans. 1846)
