daring with his life long ago, but he had left his web behind him, and that beautifully minute thread has been carrying off to the earth a portion of the electric fluid, before it had been received, and tested, and registered, by the mechanism below. Such facts show the exceeding delicacy of the observations. For seven years, the magnets suspended in this building were constantly watched every two hours—every even hour—day and night, except on Sundays, the object being that some light might be thrown upon the laws regulating the movements of the mariner’s compass; hence, that whilst men became wiser, navigation might be rendered safer. The chief observer—the _genius loci_—is Mr. Glaisher, whose name figures in the reports of the Registrar-General. He, with two assistants, from year to year, went on making these tedious examinations of the variations of the magnets, by means of small telescopes, fixed with great precision upon pedestals of masonry or wood fixed on the earth, and unconnected with the floor of the building, occupying a position exactly between the three magnets. This mode of proceeding had continued for some years with almost unerring regularity, and certain large quarto volumes full of figures were the results, when an ingenious medical man, Mr. Brooke, hit upon a photographic plan for removing the necessity for this perpetual watchfulness. Now, in the magnet-house, we see light and chemistry doing the tasks before performed by human labour; and doing them more faithfully than even the most vigilant of human eyes and hands. Around the magnets are cases of zinc, so perfect that they exclude all light from without. Inside those cases, in one place, is a lamp giving a single ray of prepared light which, falling upon a mirror soldered to the magnet, moves with its motions. This wandering ray, directed towards a sheet of sensitive photographic paper, records the magnet’s slightest motion! The paper moves on by clockwork, and once in four-and-twenty hours an assistant, having closed the shutters of the building, lights a lanthorn of _yellow glass_, opens the magnet-boxes, removes the paper on which the magnets have been enabled to record their own motions, and then, having put in a fresh sheet of sensitive paper, he shuts it securely in, winds up the clockwork, puts out his yellow light and lets in the sunshine. His lanthorn glass is yellow, because the yellow rays are the only ones which can be safely allowed to fall upon the photographic paper during its removal from the instrument, to the dish in which its magnetic picture is to be _fixed_ by a further chemical process. It is the blue ray of the light that gives the daguerrotypic likeness;—as most persons who have had their heads off, under the hands of M. Claudet, or Mr. Beard, or any of their numerous competitors in the art of preparing sun-pictures, well know.