Let the train start at a speed of a mile a minute, you would think, surely, that it must soon cross the ring.  But the minutes pass, an hour has elapsed; so the distance must be sixty miles at all events.  The hours creep on into days, the days advance into years, and still the train goes on.  The years would lengthen out into centuries, and even when the train had been rushing on for a thousand years with an unabated speed of a mile a minute, the journey would certainly not have been completed.  Nor do I venture to say what ages must elapse ere the terminus at the other side of the ring nebula would be reached.

A cluster of stars viewed in a small telescope will often seem like a nebula, for the rays of the stars become blended.  A powerful telescope will, however, dispel the illusion and reveal the separate stars.  It was, therefore, thought that all the nebulae might be merely clusters so exceedingly remote that our mightiest instruments failed to resolve them into stars.  But this is now known not to be the case.  Many of these objects are really masses of glowing gas; such are, for instance, the ring nebulae, of which I have just spoken, and the form of which I can simulate by a pretty experiment.

We take a large box with a round hole cut in one face, and a canvas back at the opposite side.  I first fill this box with smoke, and there are different ways of doing so.  Burning brown paper does not answer well, because the supply of smoke is too irregular and the paper itself is apt to blaze.  A little bit of phosphorus set on fire yields copious smoke, but it would be apt to make people cough, and, besides, phosphorus is a dangerous thing to handle incautiously, and I do not want to suggest anything which might be productive of disaster if the experiment was repeated at home.  A little wisp of hay, slightly damped and lighted, will safely yield a sufficient supply, and you need not have an elaborate box like this; any kind of old packing-case, or even a bandbox with a duster stretched across its open top and a round hole cut in the bottom, will answer capitally.  While I have been speaking, my assistant has kindly filled this box with smoke, and in order to have a sufficient supply, and one which shall be as little disagreeable as possible, he has mixed together the fumes of hydrochloric acid and ammonia from two retorts shown in Fig. 7.  A still simpler way of doing the same thing is to put a little common salt in a saucer and pour over it a little oil of vitriol; this is put into the box, and over the floor of the box common smelling-salts is to be scattered.  You see there are dense volumes of white smoke escaping from every corner of the box.  I uncover the opening and give a push to the canvas, and you see a beautiful ring flying across the room; another ring and another follows.  If you were near enough to feel the ring, you would experience a little puff of wind; I can show this by blowing out a candle which is at