260 C, F. GAUSS ON A NEW INSTRUMENT FOR OBSERVING 



SO perfect an image of the course of the disturbances as by their \ 

 combination in a single curve. A complete representation of 

 the terrestrial magnetic force (/'. e., its horizontal portion) at i 

 each moment, is given by a single straight line, of which the 

 length is proportional to the intensity, and the angle which it 

 forms with a fixed line is equal to the declination. To repre- 

 sent the force in several successive moments, the same point of 

 commencement of the different straight lines is preserved, and 

 the terminal points alone exhibited ; these are noted with the cor- 

 responding times, and are united by a line (Plate XIII, part 2). 

 The radii themselves are not drawn, and even the common point 

 of commencement, to obtain anything like a convenient scale, 

 must always be situated far beyond the drawing. This leads us to . 

 a new point of view, fi'om which we may consider such changes of 

 the two magnetic elements. In fact, they ai'e the two horizontal 

 components of that always comparatively small disturbing force, 

 to w hich the mean terrestrial magnetic force is at each moment 

 subject, resolved into two directions — one in the magnetic meri- 

 dian, and the other perpendicular to it. The second component 

 is given directly by the magnetometer, the first by the new ap- 

 paratus ; for which reason both must be reduced to a common 

 measure before the drawing. 



In applying this very illustrative mode of representation, it 

 must be remembered that the course of the changes during a 

 whole day cannot be represented in one drawing without confu- 

 sion, if there are frequent and quickly vaiying changes, as the 

 curve would present too many convolutions : it is necessary, 

 therefore, in such case, to make separate drawings for shorter 

 intervals. 



If we compare the new apparatus and the magnetometer, we 

 find that the two, with respect to some pio-poses, serve recipro- 

 cally to render each other complete ; but, in other respects, 

 have one and the same application. For the determination of 

 the absolute declination, the magnetometer alone is applicable, 

 and not the new apparatus. The changes of the declination, and 

 especially the quickly varying changes, may be followed with 

 both. For determining the absolute intensity, both apparatus 

 may be employed, although the use of the magnetometer is some- 

 what less complicated than the sole employment of the new in- 

 strument Avould be ; but the foi-mer of itself can only give the 

 mean value of the intensity for a certain interval, and the quickly 



