42 GAUSS AND WEBER ON TERRESTRIAL MAGNETISM. 



Dollars. 



10. Two mirror-holders, with adjustments and mirrors 43 



11. A weight-holder, with two half kilogrammes with 

 hooks ........•• 7 



12. A case with glass Ud ..... 16 



13. Three measm-ing scales, 6 metres long ... 4 



Weber. 

 II. 



Method to be pursued during the terms of Observation. 



The six appointed terms every year fall towards the end of the 

 months Januaiy, March, May, July, September, and November ; 

 they commence at noon, Gottingcn mean time, on the last Satur- 

 day in each of those months, and terminate at noon on the 

 following day ; the sub-tenns, which have liitherto been added 

 to the principal terms (from 8 to 10 in the evening on the Tues- 

 day and Wednesday of the following week) wiU in future be dis- 

 continued. According to the rule, the position of the magnet 

 needle is in each term determined eveiy 5 minutes, so that one 

 tenn affords 289 results. The clock is regulated, in Gottingen, 

 previous to the commencement of each term, accurately to mean 

 time. As a near coincidence in the time of the indi^^dual deter- 

 minations at various stations is highly desirable, most of the ob- 

 seners at other places are accustomed to regulate their clocks also 

 to Gottingen mean time. Where this cannot be done, it is recom- 

 mended that such whole minutes be chosen for the moment of 

 obsenation as approximate nearest to the times of obsenation at 

 Gottingen. If, for instance, it had been found, pre\aously to the 

 commencement of the term, that the clock-time required for 

 the observations was about 13' 48" in advance of Gottingen 

 mean time, the needle should be observed at 0*^ 14', 0*^ 19', 0^ 

 24', O'^ 29', and so forth of the clock. In every case, it is best 

 to choose ihefull minutes. 



The position of the needle to be determined for any instant 

 of time is not that position which the suspended magnet bar 

 actually has at the instant, but that which it would have were 

 its magnetic axis at that instant exactly in the magnetic meridian. 

 This distinction was unnecessary, as long as such needles only 

 were employed as were not susceptible of very great accuracy : 

 with them it was only requisite that the needle at the time of ob- 

 servation should not be in perceptible ^^bration, and the obsena- 

 tion might at once be made. The instruments now in use are 



