LET us begin this Friday morning on a sombre note: "The pomp of power," says Thomas Gray:
And all that beauty and
that wealth e'er gave,
Awaits alike th' inevitable
hour
The paths of glory lead but
to the grave.
Now, the grave, without a doubt, is a damp inhospitable spot, but surprisingly perhaps, and contrary to accepted wisdom, it is not particularly cold. At times, indeed, it may be warmer six feet under than it is on top.
Before we shuffle off this mortal coil, we live on the surface of the earth, where the ground responds quickly to heating by the sun and loses energy rapidly when it is absent. But just beneath the surface, the soil reacts slowly to temperature changes overhead, and the deeper down you go, the less the surrounding temperature tends to change with the passing days and weeks.
At a depth of 4 inches, for example, the difference between the average temperature of the warmest month in any year, and the average temperature of the coldest month, is about 13C. At a depth of 4 feet, this difference between the two decreases to about 7C, and at 30 or 40 feet below the ground; there is hardly any seasonal variation; the soil temperature remains more or less constant right throughout the year.
Temperatures underground are slow to vary because the layers of soil act as an efficient insulator. They make it difficult for the heat of the sun to penetrate, and also retard the loss of heat on cold winter nights. In the summer, therefore, the temperature near the surface is significantly higher than that some feet below; conversely in winter, since cooling takes place at the surface, it is normally warmer underneath.
The durial variation of temperature - the way it changes through the day - also decreases with depth. Air temperature rises during the day to reach a maximum in the early afternoon, and drops to a minimum around dawn. This pattern, albeit less marked, is detectable beneath the ground to a depth of about 4 feet, but below this level, the temperature remains more or less constant throughout the 24 hours. And not too far beneath the surface, the extremes of a hard winter are hardly felt at all.
Freezing in the winter at a depth of, say 4 inches, is not uncommon; below 8 inches it occurs only during a prolonged cold spell; but sub zero temperatures at depths greater than 12 inches are unknown in Ireland.