THE DISTURBANCE OF ISOMAGNETICS. l-')7 



by the usual method, from simultaneous equations containing time, 

 azimuth, and collimation.* For this sort of work the usual method 

 of forming normal equations by differentiation is altogether super- 

 fluous, and we follow the abridged method of Cauchy. By this 

 means we found two azimuths from two sets of observations on 

 successive nights, when weather permitted ; on other occasions, obser- 

 vations were made, one in the evening, and one early next morning. 

 The results of the two determinations usually came out within 10"; 

 this difference is due to two sources, one the error of observation, and 

 the other the error of graduation. The latter will be eliminated to 

 a o-reat extent by takino; a number of observations on some circum- 

 polar stars, and a number of readings of the azimuth circle on dif- 

 ferent parts of the graduation. For this purpose, we have published 

 in SugaJcu-butsuri-gakJcwai-h'ji., vol. V., a somewhat extensive table of 

 azimuths of Polaris, for different hour angles and latitudes, which can 

 be used for observations of either « or ^^ JJrsœ Minoris for several years 

 to come. As these two stars differ l^y nearly 6 hours in right ascen- 

 sion, it will always be possible to take observations which differ by 

 several minutes of arc in azimuth in course of half an hour. 



Magnet. — The magnet is a solid steel bar of nearly square cross 

 section polished on all sides. Its magnetic moment was found 

 to have fallen a great deal, and we therefore remagnetized it a few 

 days before starting, taking care not to reverse the previous magneti- 

 zation. Placing the magnet on a glass plane, it was found to be 

 distinctly curved. A lateral face of this magnet, which has better 

 polish than the others, has a round spot towards the north end, and this 

 face with the mark always served as the reflecting face in our vibration 

 experiments. It has remained nearly plane, while the faces perpen- 



* See reductiou sheet at the end. 



