David Rittenhouse, of Norristown, whom Thomas Jefferson considered “second to no astronomer living,” built an orrery worth a thousand dollars, to illustrate mechanically the motions of all the planets, and though the instrument is still treasured in the University of Pennsylvania, and its duplicate at Princeton, among the relics of a past age, it is assigned to the category of toys. Mural circles, much depended upon to measure the declination of heavenly bodies, have fallen into disuse, supplanted by improved transit instruments. [Illustration: THREE-INCH TRANSIT, BY WARNER & SWASEY.] XVI. PROBLEMS FOR FUTURE STUDY. Many problems are in store for the future. The field for research still opens wide. How the solar activity is to be maintained was answered by Newton in the suggestion that comets falling into it kept up its supply of matter and energy. Waterston, in 1853, propounded the thought that meteoric matter may be the aliment of the sun. Now the prevalent theory is that a contraction of the sun’s volume, constantly in progress, but so slight as to be invisible to the most powerful telescope, is competent to furnish a heat supply equal to all that can have been emitted during historic periods. Professor Newcomb answers the question, “How long will the sun endure?” by saying, “The physical conclusion to which we are led by a study of the laws of nature is that the sun, like a living being, must have a birth and will have an end. From the known amount of heat which it radiates we can, even in a rude way, calculate the probable length of its life. From fifteen to twenty millions of years seems to be the limit of its age in the past, and it may exist a few millions of years, perhaps five or ten, in the future.” [Illustration: CAROLUS LINNÆUS OF SWEDEN, FATHER OF MODERN BOTANY. This illustration was prepared by a Swedish society, and represents the famous botanist after his return from the exploration of Lapland, and with a bunch of his favorite flower (_Linnæa borealis_) in his hand. ] STORY OF PLANT AND FLOWER BY THOMAS MEEHAN, _Vice President Academy of Natural Sciences, Philadelphia_.