378 Ferret — Measures of Intensity of Solar Radiation. 



Art. XLII. — On certain Measures of the Intensity of Solar- 

 Radiation / by William Feerel. 



Where one thing in nature increases or decreases with an- 

 other, either from the relation of cause and effect, or any other,, 

 the first cannot be regarded as a true relative measure of the 

 second, unless there is strict proportionality in the increase or 

 decrease of the two. The height of the mercury in a ther- 

 mometer with reference to temperature, and in a barometer 

 with reference to air-pressure, very nearly fulfills this condi- 

 tion, but not accurately so, and it is well understood that cer- 

 tain corrections have to be applied to the heights of the mer- 

 curial columns in both cases before they can be regarded as 

 true measures of temperature in the one case, or of air-pressure 

 in the other. 



The mere differences of temperature indicated by the black- 

 bulb thermometer exposed in the open air to solar radiation, 

 and the air-temperature, without regard to absolute tempera- 

 ture or the commotions of the air, were once used as true 

 measures of the intensity of solar radiation. But it was soon 

 discovered that the uncertain effects of air- currents so changed 

 the indications, that they were worthless as measures of this 

 intensity. To obviate these effects the black-bulb was then 

 placed in vacuo in a glass inclosure, and the differences between 

 the temperatures of the black-bulb thermometer, and the air- 

 temperature were regarded as true relative measures of the 

 intensity of solar radiation, without regard to the absolute 

 temperatures. 



On account of the uncertainties of the air-temperatures, and 

 the differences between them and those of the glass inclosure. 

 an apparatus, called the Arago-Davy actinometer, consisting of 

 a black and a bright-bulb thermometer in vacuo in glass inclos- 

 ures, has been used, especially at the meteorological observa- 

 tory at Montsouris, and the differences of temperatures indi- 

 cated by the two thermometers, when exposed to the solar 

 rays, without regard to absolute temperatures, have been used 

 as true relation measures of the intensity of radiation. 



It was first shown by the writer* that, in neither case, can 

 the mere differences be regarded as true measures of the in- 

 tensity of radiation ; that not only are these differences not 

 proportional to the intensity of the radiation for the same 

 absolute temperatures, but that they differ very much for dif- 

 ferent temperature?, when the intensity of the radiation is the 



* Temperature of the Atmosphere and the Earth's Surface, Professional Paper 

 of the Signal Service, No. XIII; and Recent Advances in Meteorology; Report 

 of the Chief Signal Officer for 1885, Part II, pp.375-380. 



