220 PHYSICS. 
varying from sixteen to twenty-four degrees, generally eighteen. As the 
sun’s orbit in higher latitudes is very slightly inclined to the horizon, 
twilight there lasts longer, while at the equator itself, where the sun’s orbit 
is perpendicular to the horizon, it is shortest, the sun occupying the least 
time in traversing the eighteen degrees just below the horizon. To find the 
duration of twilight at any place and day, it is only necessary to ascertain 
how much time is required by the sun in that part of its orbit to get eighteen 
degrees below the horizon. There will be two periods of shortest twilight 
in each year at every place. In estimating the actual and not the 
astronomical length of twilight, however, it is necessary to remember that 
the varying amount of moisture in the air will influence the reflection or 
dispersion of the solar rays very materially. Thus if the particles of vapor . 
exist abundantly at a great elevation, and the lower strata of air be very 
transparent, twilight may last an uncommonly long time. Between the 
tropics twilight is always of very short duration ; in Cumana, on the coast 
of Sierra Leone, at Paramatta, &c., it lasts only a few minutes; in Chili, 
during the dry season, only a quarter of an hour, although the time’ calcu- 
lated, according to the preceding principles, would allow an hour and 
twelve minutes at the equator itself, even at the time of the equinoxes. The 
polar regions rejoice in very long twilights, at whose beginning and end the 
sun is probably thirty degrees below the horizon. 
Various Phenomena of Atmospheric Refraction and Reflection. 
The twinkling of the stars, frequently more conspicuous at one time than | 
at another, is produced by the agitation of vapor by aerial currents, and 
may justly be considered, when very remarkable, as a presage of wet 
weather; it depends upon the difference in the refraction of light in warm 
and cold, wet and dry air. It is sometimes accompanied by a change of — 
color; in the fixed stars it is incomparably more evident than in the 
planets. Stars in the vicinity of the horizon sparkle more than those nearer 
the zenith, these frequently appearing nearly quiet. The twinkling is 
greatest when currents of unequal temperature move in contact, mixing 
partially ; for this reason it is not the same in all regions, being visible 
during the dry season in the tropics only near the horizon, and but feebly 
there. In higher latitudes it becomes very striking during severe cold and 
a clear sky. | 
The so-called drawing of water is another phenomenon falling under 
this head. This (pl. 27, fig. 4) takes place when, while the sky is well 
covered with broken clouds, the sun stands behind a rain cloud and shines 
through openings in it. By the reflection of drops of water, particles of 
dust, &c., light streaks are presented, which, while apparently converging 
towards the sun, in reality are parallel. This appearance is seen more 
frequently in summer than in winter, oftener when the sun is low in the 
horizon than when he stands higher, and may generally be taken as 
indicative of approaching rain. 
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