GAUSS AND WEBER ON TERRESTRIAL MAGNETISM. 2/ 



be brought near them. It is even desirable that the entire build- 

 ing, even as to its side walls and roof, should contain no iron ; 

 but it is unnecessai-y to be so cautious as to fear placing a clock, 

 or a theodolite with steel pivots, at a distance of from five to six 

 metres from the instrument. The influence of the steel parts, 

 if they are magnetic, may be approximately deduced by calcu- 

 lation, and is found to be much too small to be sensible at those 

 distances. Small pieces of iron outside the room have still 

 less influence. If, however, there were in the neighbourhood 

 large masses of iron, especially very long iron bars, (such as 

 iron railings), although their influence would be very small, yet 

 it should not be totally neglected. If they are at a distance of 

 a hundred feet from the observatory, they offer no important 

 impediment, at least if they are fixed. Such a locality is suffi- 

 cient for measuring the Declination and Intensity, and also for 

 obsemng their changes. Measurements of the Inclination may 

 be performed in the same locaUty, but not, however, without in- 

 terrupting the other observations. It therefore appears con- 

 venient, when circumstances permit, to assign a separate locality 

 for measurements of the Inclination, which may be at no great 

 distance from the first-named room. Whei'e no absolute mea- 

 surements are made, but only the changes of Declination ob- 

 served at the fixed terms, such a room suffices, even should it 

 contain much iron within and without its walls, provided that 

 all the iron remains unmoved during the observations. The 

 room of the Gottingen magnetic obsen-atory is figured in Plate 

 II., and the ground-plan in Plate III. 



For the purpose of setting up the instruments, a line should be 

 drawn on the floor representing the magnetic meridian, which line 

 must pass nearly through the middle of the room, and terminate 

 at the southern or northern end of it, where a firm foundation 

 must be made for the theodolite and clock. When this founda- 

 tion has been prepared, and the theodolite placed upon it, let a 

 scale be first attached to the stand of the telescope, so that a 

 plumb-line let fall from the object glass of the telescope passes 

 freely before the scale. The scale must be horizontal and at 

 right angles with the magnetic meridian ; it must be cajjable 

 of being raised or lowered at pleasure, and must be bisected 

 by the magnetic meridian passing through the optical axis of 

 the telescope. Next let fall a plumb-line from the ceiling 

 to the floor, in such manner that the plane of the magnetic 

 meridian passing through this plumb-line may contain the op- 



