Account of the new Magnetical Observatory at Gottifigen. 345 



But to render this exactness complete, the apparatuses were 

 required to be made on a larger scale; and, to procure for 

 the results a perfect freedom from extraneous influences, it 

 was also necessary to erect a building entirely free from 

 iron. 



The magnetic observatory stands in an open space about 

 a hundred paces west of the astronomical one, and forms an 

 oblong square, placed due north and south, of 32 Paris feet 

 in length, by 15 in breadth, with two projections on the 

 longer sides. Of these the western forms the entrance, and 

 also serves for certain observations as an extension of the 

 space within ; while the eastern, which is perfectly parted off 

 from that principal apartment, is appropriated to the watch- 

 man of the observatory. Everything in the building usually 

 made of iron, such as locks, hinges, window linings, nails, &c, 

 is made here of copper. Drafts are as much as possible pre- 

 vented. The height of the apartment is 10 feet. The mag- 

 netic apparatus being essentially the same as that alluded 

 to above, we shall here confine ourselves to merely point- 

 ing out the differences. The magnetic bar is made of Uslar 

 cast steel, which is particularly fit for magnetic observations ; 

 and it is frequently replaced by another, both being very nearly 

 of the same size, viz. 610 millimetres in length, 37 in breadth, 

 and 10 in thickness, having a weight of nearly four pounds. 

 The mirror is 75 millimetres broad and 50 high. The suspen- 

 sion of the rod from the ceiling is effected by means of a 200- 

 fold untwisted silk thread 7 feet long ; but the circle of torsion 

 is not, as before, at the upper, but at the lower, end of the 

 thread, and so connected with the little skiff bearing the rod 

 as to turn with it. Silk threads, as has been observed in Mr. 

 Gauss's treatise, Intensitas Vis magneticce terrestris, (p. 19,) 

 have the great advantage over metallic ones that their 

 power of torsion is but small : in the present thread it is only 

 the nine hundredth part of the horizontal power of direction 

 of the magnetic rod ; whilst the twisting power of a metallic 

 thread of the same strength for bearing would be about ten times 

 as great. On the other hand, silk threads, especially if their 

 power of supporting does not much exceed the weight attach- 

 ed to them, have the inconvenience of considerably stretching 

 during the first weeks, or by a great increase of weight. This 

 inconvenience has been here removed by an ingenious suspen- 

 sion apparatus, suggested by Professor Webei*, fastened to 

 the ceiling, and by which the thread may be wound up as much 

 as necessary, without changing its place; while at the same 

 time, this apparatus itself can be shifted about the ceiling, if, 

 in the course of time, a removal should become necessary, by 



Third Series. Vol. 5. No. 29. Nov. 1834. 2 Y 



