Such a river as the Mississippi taking its source through an alluvial plain, has bends which go on increasing by the wearing away of the outer banks, and the deposition of mud, sand and gravel on the inner bank. In 1876 at the Glasgow meeting of the British Association for the Advancement of Science, Professor James Thomson showed a model which made the phenomena of the case perfectly clear. A stream eight inches wide and less than two inches deep, flowed round a bend. As it turned this bend the water exerted centrifugal force, while a thin layer of the water at the bottom, representing a similar layer close to a river-bed, was retarded by its friction with the remainder of the stream, exerting less centrifugal force than like portions of the larger body of water flowing over it farther away from the bottom. Consequently the bottom layer flowed in obliquely across the channel toward the inner bank; rising up in its retarded motion betwixt the fast flowing water it protected the inner bank from scour. At the same time this retarded current brought with it sand and other detritus from the bottom, duly deposited along the inner bank of the stream. Instrumental Aids to Observation. The powers of the eye, acute as they are, have narrow limits; inestimable therefore is the value of the microscope, the telescope and the camera which bring to view uncounted images otherwise unseen. Let us remark how in the early days of instrumental aids a great observer just missed noting a phenomenon of utmost importance,--the black lines of the solar spectrum, upon which Fraunhofer, an optician of Munich, based his spectroscope. In sending a solar beam through a lens and a prism Sir Isaac Newton admitted the rays through an oblong slit at times as narrow as one twentieth of an inch. He saw the familiar colors, from red to violet, and nothing more. Even with a crown lens, such as he probably used, four lines distinctly appear; that is, they appear to-day, to an observer who is looking for them. In 1802 these lines were observed, as far as we know, for the first time on record, by Dr. Wollaston, who drew six of them in a diagram accompanying a paper in the Philosophical Transactions. Four of these lines he regarded as boundaries of the colors of the spectrum; of the other two lines he attempted no explanation. He used prisms of various materials but found no alteration in the lines while he studied a sunbeam. When he employed candles or an electric light he found the appearances different, why, he could not undertake to explain. In 1814, Fraunhofer observed these lines in detail, mapped them, and proved that they identified elements long known to chemists. As he built his spectroscope he gave the chemist, the physicist and the astronomer an instrument of research worthy a place beside either the microscope or the telescope.