This investigation is founded upon a long and unique series of readings of deep rock temperature taken at the Calton Hill, extending from the year 1837 to the present time. The thermometers were installed under the direction of the late Professor James Forbes and were described in detail by him to this Society. They were four in number, sunk in the rock at depths of 25·6 feet, 12·8 feet, 6·4 feet, and 3·2 feet, and consisted of large cylindrical bulbs 6 to 8 inches long and 1½ to 2½ inches in diameter, connected by long, fine capillary tubes with the surface of the ground, where they expanded into tubes of wider bore having degrees of convenient length for reading. The bulbs were inserted into tin cylindrical boxes filled with plaster of Paris as a precaution against fracture, and the liquid used was alcohol. Regular weekly readings were commenced in 1837. The thermometer at depth 12·8 feet was broken by frost in 1861, but the readings of the remaining three continued without interruption till the year 1876, when a Portuguese sailor from a ship in Leith scaled the Observatory wall one night and wantonly destroyed all the thermometers and their wooden shelter.