26 GAUSS AND WEBER ON TERRESTRIAL MAGNETISM. 



obtained with them. It is true, that what the public have already 

 been made acquainted with on this subject, in former memoirs 

 and notices* 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, Dubhn, Freiberg, Greenwich, Kasan, Milan, 

 Munich, Naples, and Upsala, by Meyerstein of Gottingen ; and 

 those for Gottingen, by Apel, and those for Cracow, Leipzig, and 

 Marburg, by Breithaupt of Cassel, are almost perfectly similar. 

 The description of the smaller instiniments which have been em- 

 ployed 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 appai'atus. Nor will 

 any mention be made of larger instruments, because if they are 

 to fulfil all pui'poses, 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 w ith it, as is the case, for instance, at Gottingen, where 

 they are in the direction of the astronomical meridian, which at 

 present forms with the magnetic an angle of 18-j^ 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 pui-pose, 

 a double door, and sometimes even double windows, are neces- 

 sary ; and there must be a solid foundation, upon w hich a theo- 

 dolite and clock may be erected. It is also necessai-y that, from 

 the place of the theodolite telescope, a distant object, the azi- 

 muth of which is known or may be accurately determined, may 

 be seen through one of the windows. The floor in the neigh- 

 bourhood of the instruments, i. e. near the centre of the room, 

 must contain no iron, nor must any object containing that metal 



* In the niemoi', Intensiias vis ma/j7ieticcE /errestri.i ad metisuram ahsoluttivi 

 revocala; auctore,C. F. Gauss, Goltinga-, 1833; further, in the GUttinghchcn 

 fielelirlcn /Ji/zci(/vi>, 1832, p. 2011, 18oJ, p. ol^, and in Schumacher's 7o/??'- 

 biiche, 1836, p. 1. 



