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Rh gas, the amount of which can be measured either continuously or every hour; but in its present form the apparatus is affected by several serious sources of error.

The electric compensation pyrheliometer, as invented by Knut Angström (Ann. Phys., 1899), offers a simple method of determining accurately the quantity of radiant energy. He employs two blackened platinum surfaces, one of which receives the radiations to be measured, while the other is heated by an electric current.

The difference of temperature between the two disks is determined by a thermocouple, and they are supposed to receive and lose the same amount of energy when their temperatures are the same. A Hefner lamp is used as an intermediate standard source of radiation, and alternate observations on any other source of radiant heat give the means of determining their relation to each other.

By means of two such instruments Angström secured simultaneous observations on the intensity of the solar radiation at two points, respectively, 360 and 3352 metres above sea-level, and determined the amount of heat absorbed by the intermediate atmosphere. An accuracy of 1–1000 appears to be attainable, and this apparatus is now being widely used. The records of 1901–1905 have already given rise to the belief that there is a variation in our insolation that may eventually be traced back to the sun’s atmosphere.

Meteorograph.—The numerous forms of apparatus designed to keep frequent or continuous register of the prevailing pressure, temperature, moisture, wind, rainfall, sunshine, evaporation, and other phenomena are instruments that belong peculiarly to meteorology as distinguished from laboratory physics. Such apparatus may be broadly divided into several classes according as the records are obtained by the help of photography, or electricity, or by direct mechanical action. The prevailing tendency at present is in favour of apparatus in which the work of the recording pen is done by a falling weight, whose action is timed and limited by the making and breaking of electric currents by the meteorological apparatus proper. The most serious defect in such instruments, even when kept in good working order, is a want of sensitiveness commensurate with the desired openness of scale. It is very important that a fraction of a minute of time should be as recognizable as one-tenth of a degree of temperature; one thousandth of an inch of barometric pressure, and velocities of one hundred miles per hour, as well as rapid changes in all these elements, must be measurable. But instruments whose scales are large enough to record all these quantities are usually so sluggish as regards time that the comparison of the records is very unsatisfactory. In order to study the relationships between temporary and fleeting phenomena, it is necessary that all instruments should record upon the same sheet of paper, so that the same time-scale will answer for all.

Polarimeter.—The brightness and blueness of the sky light, and especially its polarization, have been observed with increasing interest, as it seems possible from these elements to ascertain something with regard to the condition and amount of the moisture of the air. With a simple Nicol’s prism held in the hand and turned slowly about the axis of vision one can quickly recognize the fact that the sky light is polarized, and that the polarization is largely due to the air or dust lying between us and the clouds in the distant horizon. Arago, with a more delicate form of polariscope, determined the existence of a socalled neutral region near the sun. Babinet located a neutral point or zone about as far from the anti-sun as was Arago’s from the sun itself. Brewster discovered a neutral point near the sun and horizon, disappearing when the sun is more than 15° above the horizon. Finally, Brewster explored the sky sufficiently to draw lines of equal polarization, which he published in Johnston’s Physical Atlas, and which were confirmed by Zantedeschi in 1849. Since those days far more delicate work has been done—first by Bosanquet of Oxford, afterwards Prof. E. C. Pickering of Harvard University and Prof. A. W. Wright of Yale University. A later contribution to the subject is by Jensen (see Met. Zeit. for Oct.–Dec. 1899), who has observed the brightness as well as the polarization, and thus completed the data necessary for testing the various physical theories that have been proposed for the explanation of this phenomenon. We owe to Tyndall the discovery that when a beam of white light penetrates a mass of fine aqueous mist the latter sends off at right angles a delicate blue light, which is almost wholly polarized in a plane at right angles to the plane