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Dr. E. Frankland. On Measuring the 



[Feb. 2, 



trustworthy determinations of air or shade temperatures in different 

 localities would be obtained. 



Determination of Sun Temperature. 



The term sun temperature, as commonly employed, has a very 

 vague meaning. If a body could be placed in sunlight under such 

 circumstances as to absorb heat rays and emit none, its temperature 

 would soon rise to that of the sun itself. But as all good absorbers 

 of heat are also good radiators, the elevation of temperature caused 

 by the exposure of even good absorbers to sunlight is comparatively 

 small. Thus an isolated thermometer, with blackened glass bulb, 

 placed in sunshine, will rarely rise more than 10° C. above the tem- 

 perature which it marks when screened from direct sunlight. Under 

 these circumstances, however, the thermometer loses heat not merely 

 by radiation, but also by actual contact with the surrounding cold air. 

 If the latter source of loss be obviated a much higher sun temperature 

 is obtained ; thus, the blackened bulb inclosed in a vacuous clear glass 

 globe will sometimes, when placed in sunlight, rise as much as 60° C. 

 above the shade temperature, and a still higher degree of heat may be 

 obtained by exposing to the sun's rays the naked blackened bulb of a 

 thermometer inclosed in a wooden box padded with black cloth, and 

 closed by a lid of clear plate glass. Thus I obtained with such a box, 

 on the 22nd of December, in Switzerland,* when the air was con- 

 siderably below the freeziDg point, a temperature of 105° C, and a 

 still higher temperature could doubtless be obtained by surrounding 

 the thermometer with a vacuous globe before inclosing it in the padded 

 box. These widely different temperatures, produced under different 

 conditions by the solar rays, show that such observations can be com- 

 parative only when the thermometer employed to measure them is 

 always surrounded by the same conditions. Under these equal con- 

 ditions, however, the relative solar intensities at different times or 

 places, are expressed by the number of degrees through which the 

 sun's rays can raise the temperature of any body — the bulb of a 

 thermometer, for instance — above that of the surrounding air. 

 Various instruments have been contrived, for such measurements, 

 but the thermometer with blackened bulb in vacuo is the most con- 

 venient. As indications of solar intensity, however, its readings 

 are, as I shall proceed to show, of little value if, as is sometimes 

 the case, the instrument be simply placed upon grass, or if the 

 shade temperature be not determined in immediate proximity to the 

 vacuous bulb. The following experiments, made with a blackened 

 bulb in vacuo verified at Kew Observatory, show how dependent 

 upon the nature of the surface beneath it are the indications of this 

 instrument. They were all made when the thermometer was placed 

 * " Proc. Eoj. Soc," vol. 22, p. 319. 



