Small breakages in the mercury, either from intervening bubbles of spirit or adhesion to the indices, may generally be rectified by cautiously tapping the frame of the instrument, so as to cause the mercury to unite by the assistance thus given to its superior gravity. These thermometers, when carefully made and adjusted to a standard thermometer, are strongly recommended for ordinary purposes, where strict scientific accuracy is not required. This is also the only fluid thermometer applicable for determining the temperature of the sea at depths. CHAPTER VIII. RADIATION THERMOMETERS. =82. Solar and Terrestrial Radiation considered.=--The surface of the earth absorbs the heat of the sun during the day, and radiates heat into space during the night. The envelope of gases and vapour, which we call the atmosphere, exerts highly important functions upon these processes. Thanks to the researches of Professor Tyndall, we are now enabled to understand these functions much more clearly than heretofore. His elaborate, patient, and remarkably sagacious series of experiments upon radiant heat, have satisfactorily demonstrated that _dry_ air is as transparent to radiant heat as the vacuum itself; while air _perfectly saturated_ with aqueous vapour absorbs more than five per cent. of radiant heat, estimated by the thermal unit adopted for the galvanometer indications of the effect upon a thermo-electric pile. Aqueous vapour, in the form of fog or mist, as is well known, gives to our sensation a feeling of cold, and interferes with the healthy action of the skin and the lungs; the cause being its property of absorbing heat from our person.