Calculatoria. "Then at last," said I, "the science of astronomy has wandered back to the cradle of its infancy, the soil of Chaldea. But what has become of the once so celebrated observatories of Leiden, Greenwich, the Pulkowa, etc., etc.?" They have been changed into calculatoria, as in fact they had been already for some time past. Among them are distributed the observations made at the central observatory, and these they have to work out. At the same time these calculatoria continue to be of some use to the young astronomer; having there to encounter no end of difficulties, he may learn the value of the Latin adage, Per ardua ad astera, and so grow ultimately into a hard-working and accurate observer. With regard to the practical results already obtained at the Orumiah observatory--in consequence of our knowledge of the celestial bodies having so considerably increased--I merely wish to call your attention for a moment to yonder map and the words printed underneath. I will rather not offend you by giving you any warning or advice in the matter. Tin Mines in the Moon. I followed the direction of his finger, and saw an immense "poster," on which I recognised at a glance the well-known lunar district of Tycho; of course I was acquainted with its ring mountains and the bright silver beams radiating as from a common centre; these were the words on the placard: GREATEST DISCOVERY OF THE AGE! INEXHAUSTIBLE TIN MINES IN THE MOON! WHOSOEVER MEANS TO GET RICH HAD BETTER ASSOCIATE HIMSELF WITH THE NEWLY ESTABLISHED MOON TIN EXPLORATION COMPANY, TYCHO. I had already risen from my seat in order to examine the map, and to convince myself that the words were actually there. As I turned round, Bacon must have guessed or gauged the degree of my astonishment; for he addressed me as follows: "You apparently do not believe in this kind of discoveries. Yet there is some truth in the first part of the announcement; nay, more perhaps than it is intended to convey; for those tin mines are incontestably inexhaustible, and for this simple reason, that they will never admit of being explored at all. Tin mines, however, they are. Careful observations with the great parabolic reflector provided with a hyperbolic 'oculaire' and a spectrum analysis system for the reflected rays have abundantly proved that those brilliant stripes radiating from Tycho are nothing but metallic tin. You will be less surprised to hear this when you remember that the moon has neither water nor atmosphere. So it is that metals which on our earth generally present themselves in an oxydal condition of some kind or other, on the contrary preserve their glossy surface on the moon just as with us silver, gold, and platina."