TERRESTRIAL MAGNETIC INTENSITY. 



11 



21. In the case of Needle No. 1. the magnetism being stationary, the time of 

 100 vibrations has been assumed from an observation made (in M. Arago's 

 Cabinet MapietiqueJ, 11th June 1833, as equal to 



247.==' 70 ; its log. 2.39392 



22. In the case of the " Flat" needle, a subsidiary table has been calculated 

 of the times for Paris, corresponding to the epochs when obsei-vations were made 

 elsewhere, which appear amongst the details to be given in the sequel. On the 

 11th June 1833, the log. time of 100 vibrations at Paris was . . . 2.52159 

 If, for the period from June 1832 to May 1833, we deduct .006184 (by 



Art. 18.) and for the month of May 1833, .00040 being the rate of 

 change for the cun-ent period (Art. 18.), we have a change for one 

 year, subtractive, 00724 



Tp, or Time at Paris, 



June 11. 1832, . . . Log. 2.51435 



July 11, 2.51535 



Aug. 11, 2.51635 



Sept. 11, 2.51735 



Oct. 11, 2.51835 



Nov. 11, 2.51935 



Adding .001 per month, as proposed in 



, Art. 19., we shall have nearly these 



values (neglecting trifling quantities) 



as approximations to the value of 



Log.Tp 



Proceeding sunilai-ly with the mean (and very regular) ratios of change in 

 Art. 18, we shall find 



( 1833, May 7, 2.52114 



1835, May 4, 2.53100 



Log. Tp <! ... July 20, ..... 2.53203 



... July 30, 2.53216 



... Aug. 10 2.53230 



23. The various corrections now considered being fixed, the application of 

 them, and the deduction of the horizontal intensities related to Paris as unity, 

 becomes easy. I have employed printed forms for this purpose, an-anged in 

 pages each containing 5 reductions after the foUoAving model, and bound up in 

 books. 



b2 



