530 PHYSICAL GEOGRAPHY. 



1. Halos. The simplest form of the halo is that of a white concentric ring surrounding 

 the sun or moon, a very common appearance in our climate in relation to the moon^ occa- 

 sioned by very thin vapour, or minute particles of ice and snow, diffused through the atmo- 

 sphere, deflecting the rays of light. Double rings are occasionally seen, displaying the 

 brightest hues of the rainbow. The coloured ring is produced by globules of visible 

 vapour, the resulting halo exhibiting a character of density, and appearing contiguous to 

 the luminous body, according as the atmosphere is surcharged with humidity. Hence a 

 dense halo close to the moon is universally and justly regarded as an indication of coming 

 rain. It has been stated as an approximation, that the globules which occasion the appear- 

 ance of coloured circles vary from the 5000th to the 50,000th part of an inch in diameter. 

 Though seldom apparent around the sun in our climate, yet it is only necessary to remove 

 that glare of light which makes delicate colours appear white, to perceive segments of 

 beautifully tinted halos on most days when light fleecy clouds are present. The illustra- 

 tion shows a nearly complete and slightly elliptical ring around the sun, the lower portion 

 hidden by the horizon, which was distinctly observed during the past summer in the 

 neighbourhood of Ipswich, of an extremely pale pink and blue tint. When Humboldt 

 was at Cumana, a large double halo around the moon fixed the attention of the inhabitants, 

 who considered it as the presage of a violent earthquake. The hygrometer denoted great 

 humidity, yet the vapours appeared so perfectly in solution, or rather so elastic and 

 uniformly disseminated, that they did not alter the transparency of the atmosphere. The 

 moon arose after a storm of rain behind the Castle of St. Antonio. As soon as she 

 appeared on the horizon, two circles were distinguished, one large and whitish, 44 in 

 diameter, the other smaller, displaying all the colours of the rainbow. The space between 

 the two circles was of the deepest azure. At the altitude of 4 they disappeared, while the 

 meteorological instruments indicated not the slightest change in the lower regions of the 

 air. The phenomenon was chiefly remarkable for the great brilliancy of its colours, 

 and for the circumstance that, according to the measures taken with Ramsden's sextant, the 

 lunar disc was not exactly in the centre of the halos. Humboldt mentions likewise 

 having seen at Mexico, in extremely fine weather, large bands spread along the vault of 

 the sky, converging towards the lunar disc, displaying beautiful prismatic colours ; and 

 he remarks, that within the torrid zone, similar appearances are the common phenomena 

 of the night, sometimes vanishing and returning in the space of a few minutes, which he 

 assigns to the superior currents of air changing the state of the floating vapours, by which 

 the light is refracted. Between latitude 15 of the equator, he records having observed 

 small tinted halos around the planet Venus, the purple, orange, and violet being distinctly 

 perceptible, which was never the case with Sirius, Canopus, or Acherner. In the northern 

 regions solar and lunar halos are very common appearances, owing to the abundance 

 of minute and highly crystallised spicula of ice floating in the atmosphere. The 

 arctic adventurers frequently mention the fall of icy particles during a clear sky and 

 a bright sun, so small as scarcely to be visible to the naked eye, and most readily detected 

 by their melting upon the skin. 



2. Parhelia. Mock suns, in the vicinity of the real orb, are due to the same cause as 

 halos, which appear in connection with them. Luminous circles, or segments, crossing 

 one another, produce conspicuous masses of light by their united intensities, and the 

 points of intersection appear studded with the solar image. This is a meteorological 

 rarity in our latitude, but a very frequent spectacle in the arctic climes. In Iceland, 

 during the severe winter of 1615, it is related that the sun, when seen, was always 

 accompanied by two, four, five, and even nine of these illusions. Captain Parry describes 

 a remarkably gorgeous appearance, during his winter sojourn at Melville Island, which 

 continued from noon until six o'clock in the evening. It consisted of one complete halo, 



