1^2 Observations on Twilight. [August, 
smoke, and in situations where the heavens are visible down 
to the horizon. 
Fortunately two sets of extended and systematic observa- 
tions of the evening and morning skies are in existence. 
The one series, published in the “ Zeitschrift der CEsterr. 
Gesellschaft fur Meteorologie,” was made by Dr. G. Helmann, 
in Spain, during the years 1875 and 1876. 
The author made, in the first place, determinations of the 
position of the sun at the beginning of the morning, and at 
the end of the evening twilight. He finds that the altitude 
of the sun below the horizon at the end (or the beginning in 
the morning) of astronomical twilight, though generally as- 
sumed at 1 8°, is not constant, but has a very distinct yearly 
period with a maximum in winter and a minimum in sum- 
mer. This altitude is also greater for the morning than for 
the evening twilight, and is closely connected with the rela- 
tive moisture of the air with which it incieases. T.his 
relation to moisture determines probably the difference be- 
tween the winter and the summer twilight, as well as for 
that between morning and evening. The same consideration 
will doubtless explain the fadt that the depth of the sun 
below the horizon for evening twilight is greater in high 
latitudes than in low ones. Thus at Athens the yearly mean 
is 15-9° ; whilst in the south of Spain and on the Atlantic, 
between 18 0 N. lat. and 20° S. lat., it is 15-6°. It is also 
smaller in inland districts than on the nearest seas. These 
points, however, require to be confirmed by further ob- 
servation. , 
Not less important than these measurements of the sun s 
altitude are the observations made on the physical progress 
of the twilight in which a great number of physical changes 
are combined to form a typical image. In Spain, at least, 
the optical phenomena of twilight form two distinct classes, 
that of the dry and that of the rainy season. These two 
differ, however, mainly in intensity, so that we may here 
confine our attention to one form, that of the wet season, 
as the more splendid and the more manifold. 
When the sun has still an altitude of 4 0 there appear along 
the entire horizon various faint colourations. In the west 
there is a delicate yellow of about ^ in altitude, and above 
that a light green of about T, whilst above the sun, up to 
about 50°, the sky is of a shining light blue, of a rather 
elliptical form. In the east there appears a deeper green, of 
only i° in altitude and 6o° in azimuth. 
When the sun descends to T the green on the western 
horizon reaches an altitude of 20° without becoming more 
