to the study of the most sublime and wonderful of His creations, the visible heavens. Gnomous, astrolabes, dioptras, solstitial and equinoctial armils, were placed upon the minarets of the most sacred temples. The calculations of the observer were completed in the academical institution which Moslem tradition and practice caused to be attached to every building consecrated to the worship of Allah. No profession ranked higher than that of the astronomer. The sovereign loaded him with wealth and honors. In the mosque he was received with a consideration not inferior to that exacted by the most revered expounders of the Mohammedan law. The populace, recognizing in him a mysterious personage who in secret held communion with other worlds, and too often confounding him with the astrologer, gave way as he traversed the streets, and in whispers spoke of him as the heir of the wisdom of Solomon and as a mortal invested with supernatural powers. The study of the heavens was greatly promoted by the progress made in the science of optics, and by the lucid explanation of illusions due to atmospheric refraction. In this way the twinkling of the stars, the apparent inequality of the horizontal and vertical diameters of the planets, and the prolongation of the day after sunset were accounted for. The invention of the telescope, the comparison of observations taken at widely distant stations in every portion of the globe, the perfection of apparatus which measures, weighs, and separates the component elements of our atmosphere, the intelligent application of the principles of physics, and the progressive experience of nine hundred years have not affected the definiteness and scientific accuracy of these conclusions. The Spanish Moslems possessed both terrestrial and celestial globes; some were composed of brass, others of massy silver. Their astronomical instruments were beautifully made, and were graduated with the greatest minuteness and precision. They had ten different kinds of quadrants, one of the most ingenious and complete having been invented by Al-Zarkal, of Toledo. They made use of clocks moved by water, sand, and weights. The Arabic armillary spheres and astrolabes preserved in the museums of Europe are not surpassed by the most laborious efforts of modern ingenuity in excellence of finish, and in the accuracy of adjustment which implies the possession by the artisan of a competent knowledge of the delicate operations for which they were intended. It must not be forgotten that these instruments, through whose agency such wonderful results were achieved, will compare favorably in elegance of construction with the optical appliances of the best equipped observatory of to-day.