obtained with them. It is true, that what the public have already been made acquainted with on this subject, in former memoirs and notices[1] might be sufficient; yet the perfect and accurate delineation of these instruments, which we shall give in this place, will render them easily understood, and w ill, besides, have the advantage that any clever artist can work from it with certainty. Instruments on the plan here represented have been made for Bonn, Dublin, Freiberg, Greenwich, Kasan, Milan, Munich, Naples, and Upsala, by Meyerstein of Göttingen; and those for Göttingen, by Apel, and those for Cracow, Leipzig, and Marburg, by Breithaupt of Cassel, are almost perfectly similar. The description of the smaller instruments which have been employed at some places will be here omitted, since their use has been proved to be less proper, and only to be justified when local circumstances hinder the erection of larger apparatus. Nor will any mention be made of larger instruments, because if they are to fulfil all purposes, they require a proportionally larger place of reception than has hitherto been anywhere assigned to this object.
A long quadrangular room which extends about eleven metres in the direction of the magnetic meridian, is best suited for the reception of magnetic instruments. It is not necessary that the side walls should be parallel with this meridian; they may form an angle with it, as is the case, for instance, at Göttingen, where they are in the direction of the astronomical meridian, which at present forms with the magnetic an angle of 18½ degrees. The room must be well lighted, principally from the east and west, and more particularly at the end where the theodolite or the telescope, together with the scale, are to be placed for observation. The room should be protected from currents of air, for which purpose, a double door, and sometimes even double windows, are necessary; and there must be a solid foundation, upon which a theodolite and clock may be erected. It is also necessary that, from the place of the theodolite telescope, a distant object, the azimuth of which is known or may be accurately determined, may be seen through one of the windows. The floor in the neighbourhood of the instruments, i. e. near the centre of the room, must contain no iron, nor must any object containing that metal
- ↑ In the memoir, Intensitas vis magneticæ terrestris ad mensuram absolutam revocata; auctore, C. F. Gauss, Gottingæ, 1833; further, in the Göttingischen gelehrten Anzeigen, 1832, p. 2041, 1835, p. 345, and in Schumacher's Jahrbuche, 1836, p. 1.
