perhaps, at this point, took refuge in the suggestion of non-luminosity; and here analogy was suddenly let fall.  But even admitting the central orb non-luminous, how did he manage to explain its failure to be rendered visible by the incalculable host of glorious suns glaring in all directions about it?  No doubt what he finally maintained was merely a centre of gravity common to all the revolving orbs —­ but here again analogy must have been let fall.  Our system revolves, it is true, about a common centre of gravity, but it does this in connection with and in consequence of a material sun whose mass more than counterbalances the rest of the system.  The mathematical circle is a curve composed of an infinity of straight lines; but this idea of the circle —­ this idea of it which, in regard to all earthly geometry, we consider as merely the mathematical, in contradistinction from the practical, idea —­ is, in sober fact, the practical conception which alone we have any right to entertain in respect to those Titanic circles with which we have to deal, at least in fancy, when we suppose our system, with its fellows, revolving about a point in the centre of the galaxy.  Let the most vigorous of human imaginations but attempt to take a single step toward the comprehension of a circuit so unutterable!  I would scarcely be paradoxical to say that a flash of lightning itself, travelling forever upon the circumference of this inconceivable circle, would still forever be travelling in a straight line.  That the path of our sun along such a circumference —­ that the direction of our system in such an orbit —­ would, to any human perception, deviate in the slightest degree from a straight line even in a million of years, is a proposition not to be entertained; and yet these ancient astronomers were absolutely cajoled, it appears, into believing that a decisive curvature had become apparent during the brief period of their astronomical history —­ during the mere point —­ during the utter nothingness of two or three thousand years!  How incomprehensible, that considerations such as this did not at once indicate to them the true state of affairs —­ that of the binary revolution of our sun and Alpha Lyrae around a common centre of gravity!

April 7. —­ Continued last night our astronomical amusements.  Had a fine view of the five Neptunian asteroids, and watched with much interest the putting up of a huge impost on a couple of lintels in the new temple at Daphnis in the moon.  It was amusing to think that creatures so diminutive as the lunarians, and bearing so little resemblance to humanity, yet evinced a mechanical ingenuity so much superior to our own.  One finds it difficult, too, to conceive the vast masses which these people handle so easily, to be as light as our own reason tells us they actually are.