7a 
TRANSACTIONS OF THE SECTIONS. 18 
stretching along the ecliptic, about 10° broad at its base in the horizon, and coming 
to an apex at an altitude of from 40° to 50°. It is most perceptible in the West in 
the months of February and March, at which time its apex is near the Pleiades. 
Similar appearances are presented in the morning before sunrise in the East in the 
months of August and September. The light seen in the autumn lies in the same 
direction from the sun as that seen in the spring. In the southern hemisphere the 
appearances are strictly analogous, but the times and positions of maximum visi- 
bility are, the evenings in autumn in the West, and the mornings in spring in the 
East. The portion best seen in the southern hemisphere lies in the opposite direction 
from the sun to that which is best seen in the northern hemisphere. The portion 
seen, and the degree of visibility, depend on the inclination to the horizon of the 
part of the ecliptic along which the light*stretches. The greater the inclination 
the better it is seen. At the December solstice opposite portions haye been seen 
in the northern hemisphere, one in the morning and the other in the evening; and 
in the southern hemisphere opposite portions have been similarly seen at the June 
solstice. At these seasons the ecliptic is inclined at large and equal angles to the 
horizon at equal intervals before sunrise and after sunset. The southern observa- 
tions, from which these inferences are drawn, are those made by Professor Piazzi 
Smyth at the Cape of Good Hope in the years 1843, 1844, and 1845, and published 
in vol. xx. of the ‘ Edinburgh Transactions,’ and evening observations in the autumn 
of 1848, communicated by a friend of the author resident in the interior of Brazil. 
More recently, in vol. iv. of the ‘American Astronomical Journal’ were published 
observations by Mr. Jones, a chaplain of the United States Navy, who makes the 
following statement :—“ When in latitude 23° 28’ N., the sun being in the opposite 
solstice, I saw the zodiacal light at both east and west horizon simultaneously from 
eleven to one o’clock for several nights in succession.” The ecliptic must at the 
time have nearly passed through the zenith of the observer at midnight. It is 
clear, therefore, that to be seen an hour before and after midnight, the zodiacal light 
must have extended beyond the earth’s orbit. Taking this as a necessary inference 
from the observations, it follows that the earth is either always enveloped by the 
zodiacal light, or at least when passing through the line of its nodes. Protecads 
Challis considers this to be the explanation in part of the luminosity of the sky 
which is generally perceptible on clear nights, and at some seasons in greater 
degree than at others. The American observer also states that he saw when at 
Quito, “every night, and all through the night, a luminous arch from east to west 
quite across the sky, 20° wide, and most apparent when the ecliptic is vertical.” 
This light is distinguished from the zodiacal light by its being of uniform width. 
From the ensemble of the observations, the zodiacal light is of the form of a double 
convex lens, with the sun in the centre, and the principal plane coinciding nearly 
with that of the sun’s equator. As it may be inferred from the foregoing statements 
that it envelopes the earth, we 9 conclude that it is simply dwminosity, without 
accompanying bodies. Professor Challis proposes, therefore, to account for it by 
the effect which the rotation of the vast body of the sun produces on the lumini- 
ferous medium, this effect being rendered visible by the disturbance of the gyratory 
motion by the motion of translation of the sun in space. In a similar manner, 
magnetic currents are rendered visible in the form of the aurora by the effect of 
transverse currents. This explanation he stated to be in accordance with the prin- 
ciples of the undulatory theory of light. 
he appearance of shooting-stars in the August and November periods was 
accounted for on like principles, by the disturbance given to the luminiferous medium 
by the curvilinear motion of the earth resulting from its proper motion and the 
motion of the solar system through space. At two epochs depending on the vari- 
ations of the rate of motion, and of the rate of deviation from rectilinear motion, 
the disturbances would be at a maximum, and these two epochs were assumed to 
correspond to Aug. 10 and Noy. 12. The kind of disturbance which the earth 
impresses by its curvilinear motion was supposed to be such as would produce 
eddies or whirls. Besides this, there might be a disturbance of terrestrial origin, 
analogous to that which produces the zodiacal light, which might account for the 
luminous arch noticed by the American observer. 
