Note 17 — MOUNTAIN OBSERVATORIES.

This instrument may be described as an electric balance of the utmost conceivable delicacy. The principle of its construction is that the conducting power of metals is diminished by raising their temperature. Thus, if heat be applied to one only of the wires forming a circuit in which a galvanometer is included, the movement of the needle instantly betrays the disturbance of the electrical equilibrium. The conducting wires or “balance arms” of the bolometer are platinum strips 1/120th of an inch wide and 1/25000 of an inch thick, constituting metallic antennæ sensitive to the chill even of the fine dark lines in the solar spectrum, or to changes of temperature estimated at 1/100000 of a degree Centigrade.