Seeing, then, that solar radiation plays so important a part in the production of the natural phenomena classed under the head of Meteorology, a description of the mode of estimating its amount will prove interesting, and enable the reader to realize the existence of this mighty power. M. Pouillet devised for this purpose the apparatus known as the PYRHELIOMETER, which registers the power of parallel solar rays by the amount of heat imparted to a disc of a given diameter in a given time. It consists of a flat circular vessel of steel A having its outside coated with lamp-black B. A short steel tube is attached to the side opposite to that covered with lamp-black, and the vessel is filled with mercury. A registering thermometer C, protected by a brass tube D, is then attached, and the whole is inverted and exposed to the sun, as shown at Fig. 1. The purpose of the second disc, E, is to aid in so placing the apparatus that it shall receive direct parallel rays. It is obvious that if the shadow of the upper disc completely covers the lower one, the sun’s rays must be perpendicular to its blackened surface. “The surface on which the sun’s rays here fall is known; the quantity of mercury within the cylinder is also known; hence we can express the effect of the sun’s heat upon a given area by stating that it is competent, in five minutes, to raise so much mercury so many degrees in temperature.”[2] [Footnote 2: Tyndall, “Heat a Mode of Motion.”] Sir John Herschel also designed an instrument for observing the heating power of the sun’s rays in a given time, to which the title Actinometer is given. It consists of a Thermometer with a long open scale and a large cylindrical bulb, thus combining the best conditions for extreme sensibility. An observation is made by exposing the instrument in the shade for one minute and noting the temperature. It is then exposed to the sun’s rays for one minute, and a record of the temperature made. It is again placed in the shade for one minute, and the mean of the two shade readings being deducted from the solar reading shows the heating power of the sun’s rays for one minute of time. [Illustration: 2. Herschel’s Actinometer. Scale about 1/8.] The stimulus imparted to the study of this class of phenomena by the publications of Professor Tyndall’s researches on Radiant Heat has induced a demand among Meteorologists for instruments capable of yielding more available indications than those just described. This demand has been most efficiently supplied by the ingenuity of scientists and instrument makers. [Illustration: 3. Improved Solar Radiation Thermometer in Vacuo. Scale about 1/3.]