[Illustration: 4. Solar Radiation Thermometer, black bulb and stem in vacuo, on 4 feet stand. Scale about 1/20.] The remarkable phenomenon recently discovered by Mr. Crookes, in which light is apparently converted into motion, has, at the suggestion of Mr. Strachan, received an interesting application to meteorology. The arrangement is shown at Fig. 5, where a Solar Radiation Thermometer has a Crookes’ Radiometer attached to it, which, in addition to forming an efficient test as to the perfection of the vacuum, will, it is hoped, aid in eventually establishing a relation between intensity of radiation, as shown by the thermometer, and the number of revolutions of the radiometer. The instrument has so recently been devised that any positive statement as to its usefulness would be premature; it may, however, prove a valuable auxiliary to the solar thermometer, and eventually be so far improved as to become a more definite exponent of solar radiation than the thermometer. [Illustration: 5. Radio-Solar Thermometer. Scale about 1/4.] TERRESTRIAL RADIATION. It is an established fact, confirmed by careful experiments, that a mutual interchange of heat is constantly going on between all bodies freely exposed to view of each other, thus tending to establish a state of equilibrium. It has further been ascertained that, as the mean temperature of the earth remains unchanged, “it necessarily follows that it emits by radiation from and through the surface of its atmosphere, on an average, the exact amount of heat it receives from the sun.” This process commences _slowly_ at sunset, and proceeds with great rapidity at and after midnight, attaining its maximum effect in a long night, in perfect calm, under a cloudless sky, resulting in the condensation of vapour in the form of dew, or hoar-frost, when the temperature of the surface-air is reduced to the dew-point.[3] [Footnote 3: See page 47.] The extent to which heat thus escapes by radiation under varying conditions of sky is measured by a Self-registering Terrestrial Minimum Thermometer, the bulb of which is placed over short grass, and “a thermometer so exposed under a clear sky always marks several degrees below the temperature of the air, and its depression affords a rude measure of the facility for the escape of heat afforded under the circumstances of exposure.”[4] [Footnote 4: Herschel.] [Illustration: 6. Terrestrial Radiation Thermometer. Scale about 1/6.] [Illustration: 7. Improved Cylinder Jacket Terrestrial Minimum Thermometer. Scale about 1/12.] Fig. 6 shows the ordinary spherical bulb thermometer employed for this purpose, and Fig. 7 the improved Cylinder Jacket Thermometer, which, by exposing a larger surface of spirit to the air, gives an instrument possessing an amount of sensibility in no way inferior to that of mercury.