25G C. F. GAUSS ON A NEW INSTRUMENT FOR OBSERVING 



position, and the force by which the apparatus is retained in it, 

 obey the statical laws of the composition of two forces. It will 

 now be easily seen, that if the apparatus presents the means of 

 measuring the angle between the three positions in question, the 

 relation of the two component directive forces may be calculated, 

 and consequently, we can express in absolute measure the mag- 

 netic directive force, if the force arising from the mode of sus- 

 pension is also known in absolute measure. Thus our problem 

 is solved. It is most advantageous to dispose the magnet bar 

 relatively to the other parts of the apparatus, so that it shall 

 form, in the mean position of equilibrium, nearly a right angle 

 with the magnetic meridian, to which case the term transverse 

 position is chiefly applicable. By this means, the deviation of 

 the threads ft-om their position in one plane is greatest, and the 

 result is therefore most accurate ; and a small change in the 

 magnetic declination, arising from hourly or accidental fluctua- 

 tions, has no perceptible influence on the position. On the con- 

 trary, every change in the intensity of terrestrial magnetism 

 affects the position directly, and can at once be recognised and 

 measured with the same ease, quickness, and accuracy as the 

 changes of declination are by the ordinary magnetometer. 



I had many years ago ascertained the practical applicability 

 of this mode of determination, by preliminary experiments with 

 an apparatus (very rough it is true) to which I have alluded 

 in my Memoir on Terrestrial Magnetism and on the Magneto- 

 meter. Recently, however, I have had a more perfect apparatus 

 constructed, and have suspended it in the astronomical obser- 

 vatoiy, in the spot previously occupied by the magnetometer 

 with the bar of 25lbs. weight. After what has been said, a few 

 words will suflfice for the description of this apparatus. It is 

 suspended by two steel wires 1 7 feet in length, or, to speak more 

 accurately, by a single one, the extremities of which are attached 

 to the apparatus beneath, whilst, above, the centre of the wire 

 passes over two cjdinders whicli keep it at a proper distance 

 apart (about 1^ inch) ; by this arrangement the two wires have, 

 of themselves, an equal tension. The suspension is above 

 the ceiling of the room, and the wires hang freely through a 

 circular aperture (3| inches wide) in the ceiling. The intei-val 

 of the wires, both above and below, can be increased or di- 

 minished at pleasure. The apparatus suspended to the Avires 

 consists of four principal parts. The first, to which the wires 



