PYRHE'LIOM'ETER (from Gk. rip, pyr, fire + inlios, sun par pow, met run, ure). The name given by Pouillet to an instru ment devised by him fur the purpose of measur ing the amount of heat received from the sun in a unit of time by a unit surface. This quantity is sometimes called the 'solar thermal constant.' and all apparatus expressly designed to measure the intensity of radiant heat may properly he called pyrheliometers: but, owing to the progress in our views with regard to the effects of solar radiation, it is not now generally recognized that the latter may produce either thermal, optical, or chemical effects, according to the nature of the substance upon which it falls, so that the pyrite liometer is really a special form of actinometer (q.v.) or radiometer. Pouillet's instrument, de vised in 1837. "consists of a thermometer whose bulb is inclosed in a thin that metallic box filled with water. The upper surface of the box care fully blackened is placed perpendicularly to the rays of the sun. The heating of the thermometer during five minutes' exposure to the solar action is noted, and also its cooling during five minutes when the sunlight is ent oil' by a screen. The elevation of temperature produced by the heat of the sun in five minutes, corrected for the effect of cooling or warming when the sun's rays are cut off. is to be divided by the mass of the water in the apparatus and the area of the surface; this gives the quantity of heat expressed in calories as received by a unit surface in a unit time." Tn 15S5 Professor Knut Angstrom de vised his differential pyrheliometer. composed of two identical disks of copper, each carrying a thermoelectric junction and exposed alternately to the action of the sun, a galvanometer placed in the eirenit of the two junctions measuring their difference of temperature; and in 1S93 he brought out his compensating pyrheliometer. "Two thin strips of blackened metal identical in every way are placed side by side. One of these is exposed to the rays of the sun, while the other is kept in the shade; the latter is warmed up by an electric current until its temperature is identical with that of the strip that is warmed by the sunshine. This exact equality is shown
by the fact that at this moment no thermal electric current passes between the two strips. Therefore at this moment the thermal effect of the solar radiation per unit of time is equal to that of the electric current. The intensity of the latter can easily he measured, and from it is calculated the absolute intensity of the solar radiation. A complete observation consists in making each of the two blackened strips become successively the exposed and the unexposed cal orimetric body." in 1S93 Chwolson constructed a pyrheliometer consisting essentially of two thin plates gilded on the back and blackened in front. alternately exposed to and shielded from sunshine, and whose differences of temperature can be measured many times in rapid succession by thermo-electric methods. Chwolson's ap paratus has been adopted by the Russian meteorological service, while Angstrom's appara tus has commended itself to the Weather Bureau of the United States. C'rova's mercury pyrhe liometer consists of a mercurial thermometer bulb carefully blackened and receiving the solar rays through a narrow aperture of known area. The bulb is alternately shaded and exposed several times in succession and the differences be. tween the readings give the correct effect of the sunshine. When the bulb has been properly cali brated and its water equivalent is known. its changes of temperature can be converted into calories and the instrument becomes an absolute ac•tinometer. The results of the best work that has been done with pyrheliometers give for the value of the solar thermal constant at the mean distance of the earth from the sun 4.0 calories per minute per square centimeter. hut in general all such figures are affected by tan- ignorance of the absorption by the earth's atmosphere. A very complete review of pyrheliometers is given in the report on radiation by Jules Violle, pub lished in the report .of the meeting at Saint Petersburg, ]S99, of the International Meterologi cal Committee.