Professor Hennessey, in 1857, _found the entire mass of rock and
hill on which the Armagh Observatory is erected to be slightly, but
to an astronomer quite perceptibly, tilted or canted, at one season
to the east, at another to the west_. This he at first attributed to
the varying power of the sun’s radiation to heat and expand the rock
throughout the year; but he subsequently had reason to attribute it
rather to the infiltration of water to the parts where the clay-slate
and limestone rocks met, the varying quantity of the water exerting
a powerful hydrostatic energy by which the position of the rock is
slightly varied.
Now Armagh and its observatory stand at the junction of the mountain
limestone with the clay-slate, having, as it were, one leg on the
former and the other on the latter; and both rocks probably reach
downwards 1000 or 2000 feet. When rain falls, the one will absorb
more water than the other; both will gain an increase of conductive
power; but the one which has absorbed most water will have the greatest
increase, and being thus the better conductor, will _draw a greater
portion of heat from the hot nucleus below to the surface_--will
become, in fact, temporarily hotter, and, as a consequence, _expand
more than the other_. In a word, _both rocks will expand at the wet
season; but the best conductor, or most absorbent rock, will expand
most, and seem to tilt the hill to one side; at the dry season it will
subside most, and the hill will seem to be tilted in the opposite
direction_.
The fact is curious, and not less so are the results deducible from
it. First, hills are higher at one season than another; a fact we
might have supposed, but never could have ascertained by measurement.
Secondly, they are highest, not, as we should have supposed, at the
hottest season, but at the wettest. Thirdly, it is from the _different
rates_ of expansion of different rocks that this has been discovered.
Fourthly, it is by converse with the _heavens_ that it has been made
known to us. A variation of probably half a second, or less, in the
right ascension of three or four stars, observed at different seasons,
no doubt revealed the fact to the sagacious astronomer of Armagh, and
even enabled him to divine its cause.
Professor Hennessey observes in connection with this phenomenon,
that a very small change of ellipticity would suffice to lay
bare or submerge extensive tracts of the globe. If, for example,
the mean ellipticity of the ocean increased from 1/300 to 1/299,
the level of the sea would be raised at the equator by about 228
feet, while under the parallel of 52° it would be depressed by
196 feet. Shallow seas and banks in the latitudes of the British
isles, and between them and the pole, would thus be converted into
dry land, while low-lying plains and islands near the equator
would be submerged. If similar phenomena occurred during early
periods of geological history, they would manifestly influence the
distribution of land and water during these periods; and with such
a direction of the forces as that referred to, they would tend to
increase the proportion of land in the polar and temperate regions
of the earth, as compared with the equatorial regions during
successive geological epochs. Such maps as those published by Sir
Charles Lyell on the distribution of land and water in Europe
during the Tertiary period, and those of M. Elie de Beaumont,
contained in Beaudant’s _Geology_, would, if sufficiently extended,
assist in verifying or disproving these views.