Peclet found, by his own experiments, that a body with any common unpolished non-metallic surface, kept by heat from within at 1° higher temperature than that of the air and other objects round it, loses heat from each square metre of surface at the rate of about nine kilogramme-water thermal units per hour, or, which is the same, of a gramme-water unit from each square centimetre per second. The mean conductivity of the three Edinburgh strata, in which Principal Forbes's underground thermometers were placed, is 2¾ grain-water units per second per square foot per 1° per foot rate of variation of temperature, as I have shown previously.