GEOLOGY. 311 



merit the quantity of water absorbed by each rock at given temperatures, and 

 whether the conductivity is exactly in proportion to the absorption. 



In illustration of the use that may be made of the tables, we would refer 

 to certain remarks made by Dr. Robinson, on a paper read by Professor Hen- 

 nessey, at a recent meeting of the British Association. The subject was 

 " The Direction of Gravity at the Earth's Surface." In alluding to certain 

 supposed local and temporary changes of level, he mentioned the following 

 curious fact : " He 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 attrib- 

 uted 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 lime- 

 stone rocks met. The varying quantity of this (water) through the year he 

 now believed exerted a powerful hydrostatic energy, by which the positxm 

 of the rock is slightly varied." With the light furnished by Mr. Hopkins's 

 experiments, we may pronounce the explanation satisfactory. Armagh and 

 its observatory stand on a hill 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 one or two thousand 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 con- 

 ductor, 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. Second, they are highest, 

 not as we would have supposed at the hottest season, but at the wettest. 

 Third, it is from the different rates of expansion of different rocks that this 

 has been discovered; had the limestones and clay-slate expanded equably, 

 or had Armagh Observatory stood on a hill of homogeneous rock, it would 

 have remained unknown. Fourth, though the phenomenon is in the strictest 

 sense terrestrial, it is by converse with the heavens that it has been made 

 known to us, A variation of probably a second, or less, in the right ascen- 

 sion 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 ; which has been confirmed as the true cause, and placed in 

 a clearer light by the experiments of Mr. Hopkins. One useful lesson may 

 be learned from the discovery to be careful to erect observatories ou a 

 homogeneous foundation. Edin. Phil. Journal. 



ON THE INTERNAL HEAT OF THE EARTH. 



In a discussion on the above subject, at the last meeting of the British 

 Association, Prof. Hennessey said, that Prof. Phillips had described certain 

 changes in the structure of sedimentary rocks, which changes he had been 

 led to attribute to the influence of heat. Heat might act in some instances 



