The thermometer employed in an investigation of this sort is ingenious though extremely simple. The ordinary maximum thermometer is not found to be adapted for the purpose. The instrument (Fig. 22) employed in the determination of underground temperatures is very much less complicated and at the same time much more accurate. The contrivance is indeed so worthy of notice that I do not like to pass it by without a few words. The thermometer with which the temperature of the earth is ascertained in such investigations is not like any ordinary thermometer. There is no scale of degrees attached to it or engraved upon it, as we generally find in such instruments. The instrument with which the temperature of the deep hole was measured was merely a bulb of glass with a slender capillary stem, the end of which was not closed. When it was about to be lowered to test the temperature of the rocks at the lowest point to which the drill had penetrated, the bulb and the tube were first filled with mercury to the top, and brimming over. This simple apparatus was attached to a long wire, by the aid of which it could be lowered down this deep hole. Down it went till at last the thermometer reached the bottom, which, as we have explained, it could not do until more than a mile of wire had been paid out. The instrument was then left quietly until it presently assumed the same temperature as the rocks about it. There could be no interference by heat from other strata, as the circulation of water was prevented by the plugging already referred to. The temperature to which the thermometer had been exposed must, therefore, have been precisely the temperature corresponding in that particular locality to that particular depth below the earth’s surface.