ON ASTRONOMY. 127 



zone, extending 15° on each side of the equator, which were visible 

 in Herschel's 20-fbot reflector, amounted to more than 5,800,000. 

 But this zone covers only about one-quarter of the heavens, and is by 

 no means that portion which is tlie richest in telescopic stars. The 

 great belt known as the Milky Way presents vast regions where the 

 minute stars are incomparably more dense than in Bessel's zone. To 

 say, ti en, that the whole number of stars visible in the telescope 

 amounts to many millions, is but an imperfect and inadequate state- 

 ment of the fact. We might still say, the half is not told us. But, 

 tlien^ turn away from the contemplation of single, scattered stars, and 

 look at the close clusters, of which there are so many in the heavens, 

 where many thousands of minute stars are so densely crowded together 

 that the very best powers of the telescope are required to separate 

 them. If they were thrown together, a million of visible stars would 

 scarcely occupy so large a space in the heavens as the sun's disc. 

 Then, from these stars, which are separately and distinctly visible, 

 look again at the "star dust," as Herschel calls it, which gives a 

 general illumination to the Milky Way, and is visible only in the 

 aggregate, and which yet dimly shadows forth tlie existence of indi- 

 vidual orbs more numerous than all that are seen. Who, then, shall 

 count the stars, or tell the number thereof? Down to this time more 

 than 100,000 have been accurately catalogued. Lalande's Catalogue, 

 republished by the British Association, in 1837, alone contains 47,390. 



Sidereal astronomy presents many points of interest. It may be 

 asked, what the stars are, and where they are, and wliether they are 

 unchangeably the same? Are they subjected to any law of motion? 

 Is there any bond of union between them ? Do they tell us anything 

 of time and space, of creative power and infinite intelligence, that we 

 shoidd not have known without them ? To some of these questions 

 we shall advert this evening. Others of them belong to after ages in 

 the world's history. 



Following out the plan which I have proposed to myself in these 

 lectures, I shall first endeavor to show how we arrive at some esti- 

 mate of the distances of the stars. I have, in the previous lecture^ 

 explained the method of finding the dimensions of the solar system, 

 in miles. The distance of the nearest fixed star is so vast that we 

 shall best express it in terms of the velocity of light. But if I tell 

 you that light moves at the rate of 192,000 miles in a single second 

 of time, you will very naturally ask me how this can be known. So 

 extraordinary a statement certainly ought not to be credited but upon 

 good and sufficient grounds. This problem is one of so much interest 

 in itself, and so needful to our progress, that I shall venture to give a 

 few moments to its illustration before proceeding to discuss the 

 distances of the stars. 



The velocity of light is known by three separate means : 1. By 

 the eclipses of Jupiter's satellites. 2. By the aberration of light. 3. 

 By direct experiment. 



Immediately on the discovery of the telescope by Galileo, in 1610, 

 his attention was drawn to the satellites of Jupiter, then lor the first 

 time seen. The)'- revolve round the primary in periods of from less 

 than two to more than sixteen days. Their periodic times were soon 



