of a second in a century; consequently, whatever may be the internal temperature of our globe, it cannot influence that of space. The radiation of solar heat from the earth's crust scarcely reaches our own satellite, whose rays have been always accounted destitute of caloric, till now that Melloni has detected the merest thermic impressions. Guarding against atmospheric currents and other sources of error, Melloni, by condensing the lunar light with a lens three feet in diameter upon his thermiscopic pile, found the needle to deviate from 0°.6 to 4°.8, according to the phase. We cannot, therefore, suppose that space is warmed by our earth; and those objections which are found to apply to heat issuing from the sun, may be urged to that of stellar emanations. Fourier,' in his investigations upon internal heat, has estimated the temperature of space at 50° C. below zero, or --58°F. This acute and profound mathematician founded his inquiries on the laws of radiation from the earth's surface, on the hypothesis that our globe was once a molten mass, now arrived at nearly the limits of cooling from its pristine state. This theory requires a few passing explanatory observations. We have mentioned the hypothesis of a central heat, which direct experiment goes far to establish. But, it may be asked, if there does exist an intense internal heat, why is it not apparent to our senses? Simply because the materials composing the crust of our globe are feeble conductors of caloric. The upper strata of the globe are warmed by the solar rays through conduction, and cooled by radiation. As we sink thermometers in the earth, they gradually fall till we arrive at a limit-the "couche invariable" of Fourier, about 100 feet in depth-where the temperature is constant. Below this line, and in proportion to the descent, the temperature rises, an increment explained by the hypothesis of "central heat." With different data, Svanberg has 1 Essai sur la Temp. du Globe, &c., Ann. de Chimie et Phys. xxvii. Oct. 1824. Pouillet's Elem. de Physique et de Météorologie, 8vo, tom. ii. p. 642; Encyc. Br. 7th Ed. vol. vi. p. 745. * Ed. New Phil. Jour. vols. v. vi.; Annal. de Chimie, xiii. 211; De la BecheMan. of Geol.; Annal. du Museum d'Hist. Nat. 1827; Pog. Annalen. xv. xxii. 146; Ed. Jour. of Sc. ap. 1832; Ph. Mag. 1830; Wahlenberg-Gilbert's Annalen. der Physik; Forbes,-Brit. Assoc. 1832, Rep. vol. i. p. 224. Bib. Univ. xliii. 367; Ed. Jour. of Sc. N. S. iii. 13. arrived at the same conclusion with Fourier. He found the temperature of space to be 58° F., upon the investigations of Lambert relative to the capacity of our atmosphere for caloric and the absorption of solar light. But these similar yet independent results, are widely different from that of Pouillet,' who, experimenting with swandown and delicate thermometers, exposed during serene nights to celestial radiation, arrived at the conclusion, that the temperature of space is so low as 140° C. or 220°F. The extraordinary difference between the deductions of Fourier and Svanberg from the purest mathematics, and the practical results of Pouillet, dispose us to doubt the correctness of the inference of the latter. Melloni' observes that stagnant air may, in the phenomena of nocturnal radiation, affect such experimental researches. 86. Whence this celestial temperature? The phenomena regarding caloric with which we are acquainted, cannot explain it, and Kepler's conjecture of the stellar radiation, is inadequate. Shall we seek the cause in the presence of an etherial fluid sui generis, existing above our atmosphere and pervading space? Of such a fluid, however, we possess no positive knowledge. It has been thought that some peculiar effects upon light are produced by it, and that it retards certain comets; but before we grant the latter supposition, it requires to be proved that the perturbations of these bodies do not arise from the attraction of primary planets. While we are pleased with the poetic fancy of the ancients, we do not accede to their speculations, "Ignis in ætherias volucer se sustulit oras; MANILIUS. 1 Elem. de Phys. tom. ii. 538; Compt. Rend. 1838, tom. vii. 53. Comp. Rend, No. 15, tom. xxiv. * Paralipomena ad Vitellionem, quibus Astron. optica traditur, 4to, 1604, xxxii. 25. CHAPTER V. 87. Colour of the Atmosphere. 88. Saussure's observations with the Cyanometer. 89. Peculiar depth of colour, and clearness at great altitudes. 90. Theory of tints at sunset. 91. Crepuscular light; coloured shadows. 92. Theory of colours universally applicable. 93. Crepuscular rays; diverging and converging beams. 94. Transparency of the atmosphere. 95. Use of the Photometer. 96. Direct solar light; suffers defalcation; reflected light. 97. Dark lines; nature of light. 98. Refraction; amount. 99. Illustrated; remarkable examples. 100. Polarization of light,-by refraction; by reflection: discovery of Malus. 101. Polarization of the atmosphere: neutral points of Arago, Babinet, and Brewster; normal and abnormal changes in their position, 102. Secondary neutral points; influence of luminosity. 103. Twilight; cause. 104 Conducive to our comfort. 105. Anti-twilight. 106. Second twilight. 107. Duration. 108. Anomalous brightness in 1831. 109. Peculiarities at the Pole. 110. Feelings produced by perpetual sunshine. 111. Powerful influence of solar beams in arctic regions. 112. Twinkling of the stars. 113. Influence of the atmosphere on life. "I love thee twilight! as thy shadows roll, JAMES MONTGOMERY, Esq. "Oh twilight! Spirit that dost render birth To dim enchantment; melting heaven with earth, A softness like the atmosphere of dreams; Thy hour to all is welcome." MRS NORTON. "Night wanes; the vapours round the mountains curl'd BYRON,-Lara. 87. The colour of our atmosphere is appreciable only in the aggregate. Like water, it is clear, transparent, and in small quantities colourless; but when viewed through its full extent, it is of a beautiful cerulean blue, varying from a blue approaching to white, to blue approaching to black, according to the zenith distance of the spot, and altitude of the observer. These changes depend upon the relative quantities of opaque vapour present at the time, the particles of which reflect chiefly blue and bluish-white rays; hence it follows, that in those regions where their amount is least, the sky is darkest, and as the eye pierces a greater quantity of these molecules when directed towards the horizon, we find that in proportion as the visual angle is diminished, the shade becomes. lighter. Sir David Brewster has proved by the polarization of light that the blue colour of the sky depends upon rays which have suffered reflexion by the entities of our atmosphere. t 88. With the view of tabulating for comparison the various shades, M. de Saussure constructed an instrument called the Cyanometer, which is divided into 53 degrees of intensity.' It is well adapted for the object, but a serious objection arises if constructed as originally directed, for two can scarcely be found exactly alike. With such an instrument, Saussure made the following observations:-On Mont Blanc, at an elevation of 15,750 feet, the colour of the sky was 39°, on the Col du Géant 31°, at Chamouni 19°, and at Geneva 22°. Two hourly observations from 4 A. M. to 8 P.M. inclusive-on the Col du Géant and at Chamouni, gave the following results: for the zenith, 15.6, 27.0, 29.2, 31.0, 31.0, 30.6, 24.0, 18.7, 5.5,-mean for the former locality, 23.6; 14.7, 15.1, 17.2, 18.1, 18.9, 19.9, 19.9, 19.8, 16.4,-mean for the latter, 17.8: for the horizon, 4.7, 7.5, 8.4, 9.7, 11.5, 7.6, 5.5, 4.7,-mean for the Col du Géant, 6.6, the observation at 8 P.M. being wanting; 5.5, 7.0, 8,3, 8.6, 9.1,9.3, 8.8, 8.4, 5.0,—mean for Chamouni, 7.8. Zenithal observations made two hourly, from 6 A.M. to 6 P.M., inclusive, at Geneva, gave the following indications, 14.7, 21.0, 22.6, 22.5, 20.4, 20.4, 16.3; mean 19.7. On the summit of the Monte Rosa on the 12th August 1819, M. Zumstein3 observed the 1 "L'entervalle entre le blanc pur, et le noir pur, s'est trouvé divise en 51 nuances." Voy. dans les Alpes, 4to Genève, 1786. 2 Ib. § 2084. § 2085. Ed. Jour. of Sc., vol. i. p. 22. cyanometer to indicate a depth of blue, of from 38° to 40°. Humboldt when passing from Corunna to Cumana, observed the cyanometer indicated a shade varying from 13° to 24°, and from 24° to 16°. In Europe, Caraccas and Cumana, the general intensity was as 14, 18, and 24 respectively. The tint is fainter at sea than on land in consequence of the greater quantity of vapour floating in the atmosphere over the ocean. 89. Travellers among the Himmalehs have remarked the deep blue colour of the sky, which is often rendered more marked by the contrast with the snow, and such is the clearness of the air among those elevated mountains, that the celestial bodies shine with peculiar splendour, and stars of the 4th magnitude require a telescope of small power to be rendered visible at noon. Thus at Zinchin in Chinese Tartary, visited by Moorcroft, at an altitude of 16,136 feet, a transitinstrument of 30 inches shewed stars of the 5th magnitude in broad day; the moon rose without those beams which usually herald her approach, and the sun shone like a fiery orb. 90. The late Professor Leslie remarks, that "no substance can disclose its inherent colour but by a sort of internal secretion or dissection of the rays of light. The mere reflexion from the surface of a solid body could never betray its tints, for when rendered more perfect by polish, it would only, like a mirror, send back unchanged the incident beams. To detect the subjacent colour, it is necessary that the particles of light should at least penetrate under the surface, and after suffering a sort of chemical separation, should be again emitted. In transparent substances, whether solid or fluid, the penetration is greater, but the mode of evolving the native colours must be still the same. The atmosphere besides dispersing internally the blue rays, likewise reflects in various proportions the white light unaltered. The fact is established by some experiments of polarization, which shew that such simple reflexions are the most copious from the portion of the sky which is 90° from the sun, and regularly decline on either side to the opposite points, where they cease altogether. "The white or compound beam of light suffering in its |