346 Account of the new Magnetical Observatory at Gottingcn. 



a variation in the magnetic declination. The theodolite has 

 hitherto been standing on a very solid wooden pedestal on a 

 stone foundation ; and from its position may be seen, through 

 the northern window, one of the steeples of the town, the 

 azimuth of which has been exactly determined. A delicate 

 vertical line on the opposite northern wall is the only mark 

 by which the unmoved position of the theodolite is rectified. 

 A scale , 4 feet long, divided into millimetres, is used in 

 common; but for some observations it is changed for one 

 2 metres long. The value of a division of the scale is 

 21"*3. For night observations the scale has been hitherto 

 strongly lighted with wax candles; but in future Argand 

 lamps will be used instead. One of the principal uses of the 

 apparatus now is to determine with the utmost exactness the 

 magnetic declination, and its variations in different hours of 

 the day, months, and years. The noting takes place twice 

 every day at fixed hours, viz. at 8 in the morning and 1 p.m., 

 with which times, in a regular course of daily variations, the 

 smallest and the greatest declination, at least during the 

 first months of the year, pretty nearly coincide. This man- 

 ner of noting exactly at the same hours, (instead of waiting for 

 the minimum and maximum of every day,) has been preferred 

 for important reasons, which need not be entered upon in this 

 place. The noting was begun as early as the 1st of January, 

 but because at first the thread of suspension, being too weak, 

 required, in consequence of its gradual stretching, to be fre- 

 quently wound up, which produced rather considerable vari- 

 ations in the zero point of torsion, — at first not sufficiently 

 noticed, — the [observations of the] first two months and a 

 half have been excluded. The mean values of the western 

 declination of the magnetic needle have since been as follows: 



March, second half 



April 



May 



June 



July 



8 o'Clock a.m. 



18° 38' 16"*0 

 36 6*9 



36 28 -2 



37 40-7 

 37 57*5 



1 o'Clock p.m. 



18°46'40H 



47 3-8 

 47 15-4 



47 59-5 



48 19-0 



Moreover, on certain days of the year the variations of the 

 declination are observed uninterruptedly for 44 hours, at 

 short intervals. For this purpose those days were chosen 

 which several years ago were fixed on by M. de Humboldt, 

 and on which by agreement similar observations are noted 



