REFRACTION.] 



METEOEOLOGY. 



1147 



amount of moisture present in air varies at different 

 times of the day. There appear to be two maxima and 

 two minima. The first maximum occurs about 9A.M., 

 the second at 9 P.M. ; the first minimum shortly before 

 sunset, the second about 4 A.M. Popularly, the air is 

 gaid to be most damp at sunrise ; and in the sense of dew 

 or palpable moisture, the popular expression is correct ; 

 but, provided the air be hot enough, we have already 

 seen that it can absorb large quantities of moisture, re- 

 tain it invisibly, and impart no sensation of moisture ; 

 indeed, pure steam is no more wet than a pure gas. 



Monthly Variation of Atmospheric Moisture. The fact 

 needs no comment, that all months throughout the year 

 are not equally moist. It appears that at London, Paris, 

 Geneva, and Great St. Bernard, the absolute amount of 

 vapour in these places attains its maximum in January, 

 and its minimum at the end of July or the beginning of 

 August ; but the relative moisture is greatest at London, 

 Paris, and Geneva, in December, and least in May. 



Lake the true aerial atmosphere, atmospheric aqueous 



vapour continually varies as to its amount of tension or 

 elasticity. According to Dove, the amount of tension is 

 less during north and south winds than during eastern 

 and western winds, an observation which has also been 

 confirmed by Kaemtz. Necessarily, too, the direction 

 whence the wind blows must influence the quantity pre- 

 sent of aerial vapour. North and north-east winds are, 

 at least in these latitudes, less moist than winds blowing 

 from the opposite direction. Mr. Glaisher found, during 

 his balloon ascents in 16C2, that the aqueous vapour 

 rapidly decreased as increased elevation was attained 

 (See page 1174). 



Inasmuch as the aqueous vapour dissolved invisibly in 

 air, assumes the condition of vapour whenever the air is 

 cooled below the dew-point, the influence which mountain 

 I ranges exercise in robbing winds of their moisture and pro- 

 ducing rain, will be readily evident. By an extension of 

 the same reasoning, it is evident that winds which have 

 readied into continents far distant from the ocean, and 

 lost considerable bulks of water, must be necessarily dry. 



CHAPTER III. 



* 



ATMOSPHERIC PHENOMENA {Continued). 



PHENOMENA OF ATMOSPHERIC REFRACTION. The 

 amount of atmospheric refraction is due to inequality of 

 the density of the air. But inequality of density, and 

 therefore inequality of refractive power, may be the re- 

 suit of varying amounts of expansion, referable to the 

 operation of varying degrees of heat ; and thus arise what 

 may be teamed the abnormal effects of atmospheric re- 

 fraction. 



Kvery one must have noticed the peculiar, tremulous 

 condition of the air in summer-time over an ignited 

 brick-kiln, or near a red-hot bar of metal, or even on the 

 surface of the ground, provided the weather be atifli- 

 cieutly hot. Tliis tremulous appearance is referable pri- 

 marily to the expansion of the air near a hot surface, and 

 immediately to the diminished refrangibility attendant 

 on such expansion. These local sources of heat set up 

 local currents, each being composed of air of a different 

 density from that of neighbouring current*, whence each 

 has a different refractive power. That which an ignited 

 brick-kiln, or a glowing metal-bar, can accomplish on the 

 small scale, is accomplished on a larger wale by many 

 natural causes, giving rise to phenomena both striking 

 and delusive. Pictures of ships and towns inverted, the 

 vain semblance of lakes of water in the midst of burning 

 sands where no water really exists, a'-rial cities, spectral 

 forms of men and animals all these, and many more 

 are the phenomena of atmospheric refraction and reflec- 

 tion. 



One of the most common effects of irregular atmo- 

 spheric refraction in the twinkling of the stars. Tins 

 appearance is strictly conformable with all the teachings 

 of theory in reference to the laws of refraction, and is due 

 to the fluctuations of variously-heated currents of air. 

 When these small aerial currents, having different tem- 

 peratures, are numerous, bad weather is likely to super- 

 vene ; hence, an explanation of the increased twinkling 

 of stars before bad weather sets in a phenomenon which 

 Las been very commonly noticed. 



Extreme examples of atmospheric phenomena are, for 

 the most part, only seen in hot climates ; but there they 

 are frequent. The mirage is an atmospheric phenome- 

 non, in part attributable to refraction, and in part to re- 

 flection ; it occurs in Egypt, and gives rise to the impres- 

 sion in a stranger's mind, of a lake or tranquil expanse of 

 water, though the region is only a waste of sand. The 

 explanation of the phenomenon is this : The villages 

 throughout Lower Egypt are usually built on elevated 

 mounds ; hence the houses are to some extent raised 

 above the general level of the earth, which level becom- 

 ing intensely hot, imparts heat to the atmosphere placed 

 in contact with it, and alters the refractive power of that 



portion of atmosphere. An optical illusion now ensues 

 the lower or heated atmospheric layer assumes a trem- 

 ulous appearance, like the surface of a lake, on which 

 the images of the buildings of the village are seen re- 

 flected, whilst the direct view of the village is still evident 

 in its true position. The Egyptian mirage is so decep- 

 tive, that a stranger seeing it for the first time, can hardly 

 be convinced that the semblance of water is only an 

 optical delusion. The term miraye is peculiar to India ; 

 yet the phenomenon to which it refers is common in 

 many other hot regions, especially in Central India and 

 the Sahara. 



The visual inversion of objects, a phenomenon not at all 

 uncommon in hot places, is partly due to refraction and 

 partly to reflection for, in point of fact, the function of 

 reflection may be demonstrated to be only an extreme 

 case of refraction. 



The locale is supposed to be a hot, sandy region, and a 

 date-palm ia the object seen inverted ; the explanation of 

 which phenomenon is as follows : The eyes of the ob- 

 server being at p (Fig. 63), will first see a direct image 

 of the palm- tree by rays which come straight in the direc- 



Mg. 53. 



tion of t.!ie line h p ; simultaneously he will se an in- 

 verted image of the palm-tree. Let us examine how this 

 happens. Referring to the illustration, several parallel 

 lines will be seen, c c' o" o"'. These are intended to deno- 

 minate atmospheric layers of different amounts of density. 

 Tracing the ray of light h i, let us now examine what be- 

 comes of it. First, it impinges on the upper atmo- 

 spheric layer, which is hotter, and consequently more 

 expanded, than the next layer above ; the ray h i is, 

 therefore, refracted from the perpendicular. Passing on 

 to the next layer, it ia refracted still more from the per- 

 pendicular ; and this refractive gradation is repeated on 

 the ray h i until it arrives at m, at which point, the ten- 



