REPORT ON THE MAGNETISM OP THE EARTH. 129 



untwisted fibres of silk. The advantages, however, attending 

 this method of Professor Hansteen, I consider to be more ap- 

 parent than real ; for without determining the dip, the hori- 

 zontal force, deduced from the vibrations of the horizontal 

 needle, cannot be reduced to the force in the direction of the 

 dip ; and if the dip is determined, two instruments become ne- 

 cessary where, before, only one was requisite. 



In order to obviate the inconveniences attending each of these 

 methods, I have proposed a construction for a dipping needle, 

 by means of which the observations which determine the di- 

 rection of the terrestrial force will also give a measure of its 

 intensity. The general principle of the construction is simply, 

 that the centre of gravity of the needle should not be in its 

 centre of figure, but in a line drawn from that centre at right 

 angles, both to its axis of motion and to its magnetic axis ; so 

 that, by two observations, one with the centre of gravity up- 

 wards, and the other with it downwards, the dip, and likewise 

 the relation which the static momentum of its weight bears to 

 that of the terrestrial magnetic force acting upon the magnetism 

 of the needle, may be determined. The principles on which 

 these determinations depend, and the advantages which I pro- 

 pose from the adoption of this construction, are fully described 

 in a paper read before the Royal Society, and which will appear 

 in the Philosophical Transactions of this year. 



Professor Gauss has proposed a method of determining the 

 intensity and the changes it undergoes, by v/hich he hopes to 

 reduce magnetical observations to the accuracy of astronomical 

 ones. By the vibrations of a magnetized bar he determines the 

 product of the terrestrial magnetic intensity by the static mo- 

 mentum of its free magnetism. By introducing a second bar, 

 and by observing at different distances the joint effects of the 

 first, and of the terrestrial magnetism on this, he determines 

 the ratio of the terrestrial intensity to the static momentum of 

 the free magnetism of the first. Eliminating this last from the 

 two equations, he obtains an absolute measure of the terres- 

 trial magnetic intensity, independent of the magnetism of the 

 bar. This is a most important result, for we shall thus be en- 

 abled to determine the changes which the terrestrial intensity 

 undergoes in long intervals of time. It is, however, to be ob- 

 served, that it is only the horizontal intensity which is thus 

 determined, and that, in order to determine the intensity of the 

 whole force, another element, namely, the dip, must also be ob- 

 served ; and I fear much that the introduction of this element 

 will, in a great measure, counteract that accuracy of which the 

 methods proposed for determining the times of vibration appear 



1833. K 



