1OBTEOBOLOOY. 



METEOROLOGY. V. 



HALOS PARHELIA MIRAGE FATA MOUOANA IQNIB FATUU8 



COLOURS Or CLOUDS >>N. 



. phenomena of halo, jxirln-lin or inock tunt, and 

 r mock moon*, oro also optical i:n .-<>, and may 

 be accounted for in much tho BUIUO way aa tho rainbow. 



1'iviiuently rin^'-' of various sizes, and more or leu 



(!., ut-.-'l, iiro soon to surround tho moon; this in especially thu 



case whon tho sky is covered with li^'lit < irrtin clouds, or when 



Hit' air i> charged with vapour. This appearance results from 



.M.; deflected a* they pass tho minuto particles of 



watory valour. When these particlca are Tory small, the 



nr halo, as it is frutjiu'iitly calle.l, is of large size ; and 



us tii"y increase it contracts: bonce weather indications may 



be drawn from its appearance. When it contracts, tho particles 



are collecting into larger drops, and will probably soon full in 



the form of rain ; whon, on the other hand, it spreads, they are 



diminishing in size, clearly showing increasing drynoss of the air. 



"When fleecy clouds intervene between tho moon and the 



If we scatter a little lyoopodinra on a plat* of glaa*. and look 

 at any bright light through it, we shall Me ring* round it one. 

 what resembling the appearances we have been lieeorihiiif Th* 

 minute particles of thu >iu*t net npon the ray* of light in much 

 the same way as the watery particle* in the air do. A *Pr 

 effect is seen on looking through a piece of glass on which the 

 l.n-ath ha- <-c. I..!.-!.-.- I. 



There are several other optical meteors which most just be 

 referred to, some of which may almost be dintingoished as 

 spectral illusions. The best known of these is the mirage of 

 the desert. Sometimes, as the traveller is passing oa over the 

 arid wastes of sand, he sees in tho distance the appearance of 

 beautiful hikes with inlands nitnated in th- in. The images of 

 surrounding object* are seen inverted in the water with every 

 appearance of reality ; bat when he hastens forward to enjoy iU 

 coolness, the landscape recedes from him, or entirely vanishes. 



The cause of the illusion seems to be that tho surface of the 

 sand becomes intensely heated by the burning rays of the snn, 

 and hence the layers of air resting on it become much warmer 

 than those above. Tho rays of light are accordingly reflected, 



Fig. 14. HALOS SURROUNDING THE SUN, AND PARHELIA OR MOCK SI. vs. 



observer, the prismatic colours may sometimes bo seen in the 

 corona. These appearances are not often seen around the sun, 

 because its superior brightness prevents our looking at it. If, 

 however, wo examine it through a coloured glass, or view its 

 reflection in water, we shall occasionally meet with a similar 

 phenomenon. 



The term halo is most strictly applied to luminous rings 

 which are at times seen surrounding the sun, as shown in Fig. 

 14. The form of these varies very greatly ; usually several 

 circles and segments are combined in the most remarkable 

 manner, and at tho points of intersection parhelia are frequently 

 observed. As many as nine of these mock suns have been seen 

 at one and the same time, the appearance thus presented being 

 very remarkable. 



In this country the instances of their occurrence are but rare, 

 but oa we travel into arctic regions thy aro frequently met 

 with. They aro caused by tho refraction and reflection of the 

 solar rays from minute snow crystals floating in the air, and 

 hence are much more common in cold climates. It is recorded 

 that in Iceland, during the severe winter of 1615. the sun was 

 always accompanied by some of these appearances. Minuto 

 spicnlcs of ice have sometimes been observed falling during the 

 continuance of the halo, and the form it assumes probably 

 depends upon the structure of these crystals. Similar appear- 

 ances are sometimes seen around the moon, and aro known as 

 parosolciur. 



154 X.E. 



and bent out of their course. If we look at a distant object 

 along the surface of a red-hot poker, the rays of light will be 

 similarly acted npon, and we shall sometimes be able to i 

 inverted image of the object we are looking at. 



In northern latitudes inverted images of ships have been ; 

 in the air, and even been identified ; by comparing logs it has 

 afterwards been found that their distance from one another was 

 so great that they would have been quite hidden by the earth's 

 curvature, had it not been for this remarkable change in the 

 refractive power of the air. An instance is also on record in 

 which the coast of Picardy was quite distinctly seen from 

 Hastings, appearing to be only a few miles distant, though 

 under ordinary circumstances it is quite invisible even with the 

 aid of the best telescopes. Similar appearances have been Men 

 at other places, but they are very uncommon. 



The phenomenon known as tho fata morgana, occasionally 

 seen in the Straits of Messina, is of a somewhat similar character. 

 The surface of the water appears to be covered with image* of 

 mountains, buildings, and various other objects ; frequent!?, too, 

 a second image is seen inverted under the other, and the whole 

 scene is almost like the creation of a dream. The air above the 

 water seems also at times to be full of these spectral objects. 

 The most probable explanation is that, owing to some peculiar 

 state of the air, the objects on the further shore aro apparently 

 brought nearer, and mode to appear in the middle of the strait*. 

 At the some time, the images are distorted or multiplied, so that 



