SOLAR RADIATION THERMOMETER, OR BLACK BULB THERMOMETER. This is essentially an or dinary mercurial thermometer, whose bulb is covered with a thin layer of non-reflecting and almost perfectly absorbing lampblack. Under the assumption that the black bulb thermometer absorbs and radiates more easily than the plain glass bulb, and especially more so than a polished silver bulb, the difference between the readings of two such thermometers exposed to the sunshine will depend primarily upon the intensity of the radiation received by them. In order to meas ure the solar radiation, it is necessary, first, to diminish as much as possible the conduction and convection of heat to or from thermometer bulbs.
They are, therefore, inclosed in large thin glass bulbs within which there is as perfect a vacuum as it is possible to make. It is also necessary to cut off as much as possible of radiation from the surrounding objects, and especially reflected sunshine by means of screens. Under these con ditions the solar radiation thermometer, or the so-called Arago-Davy aetinometer may be used in two ways: (1) The static method. The ther
mometers are allowed to attain the highest tem peratures they can in the full sunshine; the dif ference between their readings expresses approxi mately the intensity of the solar radiation, the exact value of which may be more exactly com puted by the method and formulre given by Pro fessor William Ferrel in his memoir "Tem perature of the Atmosphere and Earth's Sur face," pages Professional Papers of the Signal Service No. 13 (Washington, 1SS4). (2) The dynamic method. The bright and black bulb thermometers are successively exposed to the full sunshine and shaded from it a minute at a time, so that one may determine time initial rate of in crease of temperature in the sunshine and time rate of cooling in the shade. From these rates the solar radiation is calculated by the formulas first applied by Ponillet to his actinometer.