4 
F. A. P. Barnard on the Zodiacal Light. 399 
For days—it may be said weeks—after, the whole district of 
country disturbed by the principal shocks was visited by tremors. 
At Santiago the times of four were noted on the 3d; only one 
on the 4th; two or three on the 5th; and so on up to the 20th ; 
indeed, for several months their occurrence was more frequent 
than during the same period of the preceding year. Havi 
passed from the afternoon of the 6th at Aguila, the hacienda 
a friend within the deep bay of the mountains, there were oppor- 
tunities to experience some of them in the open fields. 
ng 
of 
Arr. XLIV.—Supplementary Note to the article on the Theory 
which attributes the Zodiacal Light toa Nebulous Ring sur- 
rounding the Earth ;* by F. A. P. Barnarp, LL.D., Professor 
of Mathematics and Astronomy in the University of Mississippi. 
Prof. Dana,—Will you allow me space for a few words supple- 
Mentary to the article on the zodiacal light, published in the 
March number of the Journal of Science. In that article the 
8eographical limits of visibility of the cusps of a ring encircling 
the earth, lying in the plane of the ecliptic, illuminated by the 
Sun and interrupted by the earth’s shadow, are assigned for the 
Moments when the sun is eighteen degrees below the eastern or 
Western horizon. It is true, however, that there are certain limits 
of distance from the earth’s centre, between which, if such a 
ting be situated, a certain portion of the part of it illuminated 
May be visible, under the circumstances supposed, as an ilumin- 
ated arch, though the cusp may not be above the horizon. 
ticle referred to, where AOB is the earth, 
HZR the imaginary spherical surface of 
Which the ring isa great circle, H’R’ the * 
horizontal small circle of this sphere passing ie 
through the place of observation O, S the pole of the limiting 
circle (i. e. the circle of the shadow), SQ its arc-radius, and Q 
the cusp of the luminosity, then when Q is on the horizon, the 
ring may touch the horizontal circle H’R’, or it may rise above 
it either on the illuminated or on the obscured side of Q. In the 
Ormer case, a luminous arch will be visible to the observer at 0. 
The tangency or the intersection which takes place at Q will be 
determined by the value of the angle at that point; contact oc- 
curring when this angle is ninety degrees, and the intersection 
favoring the visibility of the light, when the angle is greater than 
inety. 
Hinet 
* This volume, p. 217. 
