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.