CHAPTER III. 44. Solar Beams; physical properties. 45. Temperature; diurnal variations. 46. Mensual and annual temperatures; days of maxima and minima. 47. Ascent of Gay Lussac and Biot. 48. Temperature decreases with altitude. 49. Theory. 50. Loss of heat in passage of solar rays. 51. Rate of depression with ascent. 52. Climate. 53. Snow-line within the Tropics. 54. Modifying circumstances. 55. Snow-line in various latitudes. 56. Peculiarities on Volcanoes. 57. Snowline among the Himmalehs, highest on northern side. 58. Explained. 59. Actinism. "O thou that rollest above, round as the shield of my fathers! Whence are thy beams, O Sun! thy everlasting light? Thou comest forth, in thy awful beauty, and the stars hide themselves in the sky; the moon, cold and pale, sinks in the western wave. But thou thyself movest alone: who can be a companion of thy course! The oaks of the mountains fall: the mountains themselves decay with years; the ocean shrinks and grows again: the moon herself is lost in heaven; but thou art for ever the same; rejoicing in the brightness of thy course. When the world is dark with tempests; when thunder rolls, and lightning flies; thou lookest in thy beauty, from the clouds, and laughest at the storm. But to Ossian, thou lookest in vain; for he beholds thy beams no more; whether thy yellow hair flows on the eastern clouds, or thou tremblest at the gates of the west. But thou art perhaps like me, for a season, and thy years will have an end. Thou shalt sleep in thy clouds careless of the voice of morning,-Exult thou, O Sun, in the strength of thy youth!"" OSSIAN, Carthon. 44. The chief source of terrestrial heat is the sun, which among the heathen idolaters, especially among the ancient Persian followers of Zoroaster, under the symbol of fire,' and in the once magnificent kingdom of the Incas, received divine worship, Herod. lib. i. cap. 131; Hyde-De Relig. Vet. Persarum. pp. 10-14. "Glorious orb the idol Of early nature, and the vigorous race Of undiseased mankind!... Most glorious orb! that wert a worship, ere 5 Manfred, Act iii. Sc. ii. His beams are not only luminous but calorific,' and in addition, chemical or actinic,2 and probably magnetic. These rays may be refracted through diaphanous prisms and separately received. They possess the curious property of dissimilar refrangibility; and according to Seebeck' and Melloni, prisms of different substances produce remarkable changes in the calorific spectrum. According to Forbes," the thermal rays of the sun, before reaching our atmosphere, consist of two kinds, of which one is easily absorbable; the other cannot be absorbed, in its passage through our atmosphere; respectively as 4 to 1 of the whole number. But independent of luminous, thermal, actinic and magnetic (which however may be induced) rays, the sunbeam contains phosphorescent rays, under which certain chemical substances, e. g. sulphuret of lime, 1 Scheele-On Air and Fire, Foster, 1780; The Abbé Rochon-Opusc. 1783; Sir W. Herschel-Ph. Tr. 1800; Sir H. Englefield in 1801-Jour. Roy. Inst. v. i. p. 203; Sir H. Davy; Sir John Leslie-Inq. into Nat. of Heat; Berard of Montpellier-Mém. d'Arcueil, tom. iii., Ann. de Chimie, 1813, tom. 85, p. 309, Ann. Phil. 1813; Professor Wünsch, in 1807—Mag. der Gesellsch; Dr Seebeck of Berlin-Ed. Jour, of Sc. ii. 358; Biot-Tr. de Phys. tom. iv. p. 600; Melloni-Annal. de Chimie, 1831, 388; Sir David Brewster; Professor Powell-Rep. Brit. Assoc. 1832, vol. i. 2 Ritter of Jena-Nicholson's Sc. Jour. viii. 216; Beckman; Berthollet; Scheele -Tr. de l' Air, &c. 66; Davy-El. Chem. Philos.; Berard-An. Phil. ii. 1813; Wollaston-Ph. Tr. 1802; Wedgewood; Gay Lussac and Thenard; Senebier; Dr T. Young-Nat. Phil. v. ii. p. 647; Somerville-Ph. Tr. 1826, ii. 136; Christie-Ib. 1826, 1828; Sir J. Herschel-Lond. and Ed. Phil. Mag. 1832, 58; Daguerre; Fox Talbot; Arago; Hunt-Researches on Light; Saguez; Becquerel; Reiss and Moser-Brewster's Jour. v. ii. p. 225, Annal. de Ch. et de Phys. 1829, Poggend. Annal., Scient. Memoirs, vol. iii; Fizeau-Comptes Rendus, Nov. 7, 1842; Litterrat. der chemischen lichterstrahlen-Fortschritte der Physik, im Jahre 1845; Dargestellt von der Physikalischen Gesellschaft zu Berlin-Redigirt von Dr G. Karsten, 1847. 3 Morichini in 1813-Ann. de Ch. et de Phys. iii; Somerville-Ph. Tr. 1826, ii. 132; Christie, Ib. 1826; Baumgartner of Vienna-Zeitscrift. i. 263; Barlocci; Zantedeschi of Pavia-Ed. Jour. of Sc. 1830, N. S. 5. 76; Ridolfi at Florence; Carpi at Rome; Reichenbach. Ed. Jour. of Sc. i. 358, Oct. 1824. s Brewster Brit. Assoc. Rep. ii. 294, 1832. Roy. Soc. May and June 1842. spread on paper, glow when the violet ray of the solar spectrum is made to fall upon them. This result is not owing to the chemical rays present, for the latter may be transmitted through a medium which obstructs the passage of the phosphorescent beams. Others may exist though yet unknown or very obscure, e. g. the parathermic rays of Herschel, which, with distinguishing peculiarities, combine the actions of light and heat. The independent existence of these rays may be proved, by their separate effects when certain media are interposed. Thus a bright yellow solution, e. g. bichromate of potash, admits all the luminous, but obstructs the actinic rays; while the latter easily pass through the deep blue solution of ammonio-sulphate of copper, and scarcely a ray of light is transmitted. Melloni, with green glass and plates of alum, separated all the calorific from the luminous beams, and with black mica or obsidian, withheld the passage of the latter without imposing an obstacle to the transmission of the former. 45. The temperature of our atmosphere, or the amount of caloric' diffused in it, is constantly varying, being altered by an almost infinite number of circumstances; thus, altitude, latitude, longitude, horary and seasonal periods, the presence of aqueous meteors, winds, and the physical relations of the locality; even the aspect of the horizon, whether it is rugged or unbroken by lofty eminences, the chemical nature and colour of the soil, influence the result. As the sun advances to the meridian, we feel a corresponding increase of heat, which again declines soon after culmination. Were more required than our own sensations of warmth and cold, to prove the fact, the horary tables collected by indefatigable observers would abundantly testify. Subject, however, to numerous fluctuations, we nevertheless observe, that there are two stated periods of the day when the temperature reaches a maximum and minimum. The warmest period is generally from two to 1 Caloric properly means the active agent in producing heat. Heat the sensation of caloric. Brit. Scientific Serials, passim; Observ. of Roy. Soc. in Athen.; Tables of Kämtz,-Meteorologie; Lamont, at Munich, 1841,-Annal. de Météorologie; Beobachtungen zu Prag. 131; Brewster,-Ed. Roy. Soc. Trans. vol. x., North Brit. Rev. No. x. three hours after the sun has passed the meridian, the difference depending upon season; the coldest is nearly an hour before sunrise. A mean may be conveniently obtained by recording the oscillations of the thermometer at suitable intervals, and thus the toils of hourly observation dispensed with. Thus the sum of the indications at 6 A. M., 2 and 10 P.M. or, as suggested by Schouw, at 7 A. M., noon, and 10 P. м, divided by 3, will give a near approximation to the diurnal temperature. From observations made by Professor Dewey1 in North America, it would appear that the result obtained at the homonymous hours of 10 A. M and 10 P. M. afforded an approximation of 5-100ths of a degree to the true mean diurnal temperature. Though the maximum temperature occurs some time after the sun has culminated, we find that proximity to coasts accelerates the epoch; and in tropical regions the sea-breeze still farther modifies the result. The same has been found on mountain tops; nor is it at all remarkable: for, let it be observed, that in the plain, not only is the atmosphere warmed by direct solar rays, but by those radiated from the ground; upon the mountain top there will be a greater absorption of caloric and less radiation, more heat being abstracted by conduction there, than below, where the temperature of the earth is higher and more equally diffused. The temperature, then, upon those lofty spots, will depend chiefly upon the sun's direct calorific action, and as those rays are most powerful which pierce the thinnest atmosphere, i. e. when the sun is on the meridian, the hottest period of the day will be at, or very soon after, culmination. From hourly observations made at Leith Fort during the years 1824-1827, of which only two series, 1824 and 1825 have been published by Sir David Brewster in the Trans. Edin. Royal Society, we find that the hours of mean temperature, morning and evening, were respectively 9h. 13m. A. M. and 8h. 26m. P. M., 1824 ;9h. 13m. A. M. and 8h. 28m. P. M., 1825;-9h. 7m. A. M. and 8h. 27.25m. P. M., 1826;—9h. 12m. A. M. and 8h. 23m. P. M., 1827;-mean of all the observations, 9h 11.75m. A. M. and 8h. 26m P. M. Although 1 Ed. Phil. Jour. voi. vi. p. 352. an advance on these hours is observed at Padua, still it is worthy of observation, that while the interval between the diurnal means, or critical interval, at the former station amounts to 11h. 14.75m, that at the latter is 11h. 11m; a similar constant has been noticed in Denmark, at Apenrade,' and elsewhere. Thus the critical interval at Philadelphia, is 11h 20m.; at Inverness, 11h 13m.; at Rothesay, 11h. 7m.,— from an hourly register kept for twelve consecutive years by the late Robert Thom, Esq. of Ascog, and communicated to Sir David Brewster,' the mean temperature (47.°46 F.) occurring at 8h. 32m. A. M. and 7h 39m. P. M. ;—at Kingussie, 10h 44m.; at Belleville, 11h 14m.; at Tweedsmuir, 11h 15m.; at Plymouth, 11h.; at Kandy, 11h.; at Madras, 10h; at Colombo, 10h 55m.; at Trincomalee, 11h 5m. From the Leith observations another law may be deduced, viz.—that the mean of two homonymous hours, or hours of same name before and after midday, differs from the mean annual temperature, on an average of 0.24° F.; the maximum difference being 0.421°, and the minimum, 0.059°. Referring to this interesting subject, Sir David Brewster3 says," If we measure and represent by lines the mean temperature of every hour of the revolving year, we cannot recognise, in our own climate at least, any trace of a law which regulates the daily progression of heat. For weeks and months the hourly variations are of the most capricious and irregular character; the thermometer being sometimes stationary for a day, sometimes highest at midnight, and lowest at noon. When we combine, however, the 365 observations at each hour, by taking their mean, we find that these hourly means are the ordinates of a curve of beautiful regularity, each of the four branches of which do not differ much more than a of a degree of Fahrenheit from parabolas !" 46. Having obtained the mean daily temperature, that of the month is easily computed by arithmetical rule, the sum of the diurnal indications divided by the number of days giving the required quotient. And it is not a little remark 1 Schouw, Beiträge zur vergleichenden klimatologie 1827. North Brit. Rev. No. x. vol. v. p. 499. 3 Ib. p. 455. |