Notes on Star-Streams. 3 



zenith, crossing the horizon towards the south-east and north- 

 west. Its brightest part is low down towards the last named 

 quarter, where the constellation Cygnus lies close to the 

 horizon. One month later, the Milky Way crosses the horizon 

 towards the south-south-east, and north-north-west, and is 

 bowed some 20° from the zenith towards the west-south-west. 

 Cygnus is now half-set. Yet another month, and the Milky 

 Way is found crossing the northern and southern points of the 

 horizon, and bowed about 40° from the zenith towards the 

 west — Cygnus more than half set. At the Vernal Equinox, the 

 Milky Way crosses the horizon towards the south-south-west, 

 and north-north-east, and is bowed upwards of 50° from the 

 zenith towards the west-north-west. One month later (that is 

 about the 20th of April), and we find the Milky Way crossing 

 the horizon towards the west somewhat southerly, and towards 

 the east somewhat northerly, and only raised about 20° above 

 the northern horizon. It is now easy to follow the remaining 

 changes without special comment. The eastern end travels 

 southwards, the western northwards, along the horizon, the 

 central part approaching the zenith, just as hitherto it has been 

 seen to leave the zenith. The bright parts in Cygnus and 

 Aquila are more and more favourably seen as they approach 

 the zenith, being best seen in July and August (at 10 p.m.). 

 About this time we see the southern portion of the Milky Way 

 somewhat beyond Antares (the heart of Scorpio), whereas six 

 months before the greatest range on the opposite side (including 

 nearly the whole of the gap in Argo) had been visible. At the 

 end of October the Milky Way (at 10 p.m.), is seen vertically 

 overhead, and crossing the horizon towards the east and west 

 quarters, the western half being the most conspicuous. 



Galileo was the first to prove, though earlier astronomers 

 had entertained the notion, that the Milky Way is composed of 

 a vast number of stars, crowded closely together. But no 

 attempt was made to offer a theory of its structure until in 

 1754, Thomas Wright, in his " Theory of the Universe," pro- 

 pounded views closely according with those entertained at the 

 present time. This philosophic observer, having examined a 

 portion of the galaxy with a reflecting telescope, only one foot 

 in focal length, came to the conclusion that our sun is in the 

 midst of a vast stratum of stars ; that it is when we look along 

 the direction in which this stratum extends, that we see the 

 zone of light constituting the Milky Way ; and that as the line 

 of sight is inclined at a greater and greater angle to the mean 

 plane of the stratum, the apparent density of the star- grouping 

 gradually diminishes. 



But it is to Sir W. Herschel, and the supplementary labours 

 of Sir J. Herschel, that we owe the more definite views at 



