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THE BOLOMETER AND ITS ADJUNCTS.

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1. General view. 2. Cross section. 3. End view. 4, 5, 6, 7. Details of balancing mechanism and connections. 8. The bolometer proper. 9. Diagram of electrical connections,

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FIG. 1.-BOLOGRAPHIC ENERGY CURVES OF THE SOLAR SPECTRUM OF A 60° FLINT-GLASS PRISM. * Means shutter interposed to give zero of radiation.

† Means diaphragm introduced to give suitable height to curves.

correctly measure the relative intensities of rays of all wave lengths, whether visible or not.

The indications of the bolometer may be automatically observed by photography, and thereby the solar spectrum may be exhibited, as in figure 1, as a sinuous line whose elevation above the base line of zero radiation gives the relative intensity of the different colored or invisible rays. The two curves shown are taken independently about an hour apart, for the purpose of studying the increase of intensity of the sun's rays of different wave lengths as their path in air diminishes in length in consequence of the approach of

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FIG. 2.-VERTICAL ATMOSPHERIC TRANSMISSION FOR DIFFERENT WAVE LENGTHS.
Upper curve for Mount Wilson; lower curve for Washington.

the sun to the meridian. From such studies the results shown in figure 2 are found. The upper curve represents the percentage transmission of a vertical column of air above Mount Wilson (elevation 1,750 meters), while the lower curve shows the less transmission for vertical rays at Washington (30 meters). The wave lengths are indicated as abscissæ. From this we see how much more loss the violet rays of wave length 0.40μ suffer than do the red rays of wave length 0.70μ in traversing the air.

In order to determine the quantity of the solar radiation, we must fix our conditions independent of the variable losses in the

atmosphere. We attempt, therefore, to make the observations in such a manner as to permit a correct estimate of the atmospheric losses, so that the result can be expressed as if the measurements were made in free space beyond the atmosphere. But of course our actual work must be done at the earth's surface.

We express solar radiation in heat units called calories. As the bolometer (pl. 2) is not of itself capable of giving true calories we have devised an instrument shown in figure 3, a standard pyrheliometer, so called. A is a chamber of nearly the dimensions of a large test tube, whose walls are hollow and adapted for the circulation of a stream of water. The stream enters at E, bathes the walls and rear of the chamber and the coneshaped receiver of rays H, and passes out at F, carrying the heat developed by solar rays which enter the chamber by the measured orifice C. At D, and D, are platinum coils adapted to measure the rise of temperature of the water due to solar heating. Knowing the weight of water flowing per minute, the rise of temperature and the area of the opening C for solar rays, their intensity is determined. As a check, heat may be produced electrically in the chamber, and the proof of the accuracy of the instrument consists in finding the known quantity of electrically introduced heat correctly measured. Another simpler instrument for everyday use is the silver disk pyrheliometer shown in

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