The Sun's Radiant Heat

Abstract

THE readers of the Comptes Rendus are aware that Pegrave;re Secchi addressed a letter to the Academy of Sciences at Paris, some time ago (Comp. Rend, tome lxxiv. pp. 26-30), tables, appears to rest on the supposition that my instruments do not furnish correct indications. “It is astonishing,” he says, “that Mr. Ericsson should find with his instrument a higher stationary temperature in winter than in summer. This (even containing a review of my communications to NATURE, published July 13, October 5, and November 16, 1871, in which he questions the correctness of the reports which I have published containing tabulated statements of the temperature produced by solar radiation. His reason for questioning the reliability of my bearing in mind the greater proximity of the sun in winter) makes me think that there must be something very singular in his apparatus, possibly making all its indications deceptive. Even under the beautiful sky of Madrid, M. Rico y Sinobas found, in December, for the solar radiation, 12div, by his actinometer, and, in June 25div, 56″.” It is not my purpose to prove the fallacy of M. y Sinobas' actinometric observations; but I think “that, there must be something very singular in his apparatus,” since in North America in lat. 40° 42′ (the latitude of Madrid is 40° 24′), solar intensity at noon during the latter part of June is 64°.5; while when the sky is clear at noon during the latter part of December, the temperature under similar atmospheric conditions, reaches 58°.7. But observations made in the morning or evening during the month of June at the hour when the sun's altitude is the same as at noon in December, show that the intensity of the radiant heat in June is only 53°.08 against 58°.7 in December. Actual observations have thus established the fact that for corresponding zenith distance, the temperature produced by the radiant heat when the earth has nearly reached perihelion, is 58.7−53.8=40°.9 higher than at midsummer. Referring to the table published in NATURE, vol. v. p. 47, it will be seen that, owing to the greater proximity of the sun, the increase of absolute intensity of solar radiation is 5°.88 Fah. during the winter solstice. Père Secchi will do well to examine the subject more carefully, and make himself better acquainted with the character of the investigations which had led to an exact determination of the temperature produced by solar radiation.