12 



PROFESSOR FORBES'S EXPERIMENTS ON 



MAGNETIC INTENSITY.— HoRizoNTAi, Needle, No. 1. 



Place 



Geneva. Botanic Garden. 



Geneva. Botanic Garden. 



Date, 



1832, Nov. 10. 



1832, Nov. 10. 



Mean Time, 



No. of Vibrations observed, . 



llh-S-S"" 



Ilh42m 



100 



100 





Arg. 



Log. 



Arg. 



Log. 



Observed Time, 

 Rate Chronometer, 



Arc, 





•{ 



24o!l4 



«= 10° -» 



w=zllO° 1 



7°.0 R. 



2.38046 

 9.99990 



9.99967 

 9.99863 



240.00 



+ 21^ 



«= 10° 1 



w=:110° i 



7°.0 R. 



2.38021 

 9.99990 



9.99967 

 9.99863 



Temperature, . . 





Corrected Time (T), 

 Time at Paris (Tp) 

 Tp 





• 



239'.! 4 



2.37866 

 2.39392 



239!o6 

 1.074 



2.87841 

 2.39392 



0.01626 

 2 



0.01551 



2 



T 



( ^ j = Intensity, 







1.073 



0.03052 



0.03102 



ft 



* a = Initial semi-arc of vibration; m = number of vibrations which reduce the semi-arc to — 



§ 3. Observations on Magnetic Intensity. 



24. It now remains to give the observations which have been made, and re- 

 duced in the way already detailed. These consist chiefly of two series. One was 

 made in the year 1832, intended to form part of a very general investigation in 

 physical geography, which I meant to pm'sue throughout a journey of several 

 years. Having been diverted from this by the opening of other prospects, the 

 series remains incomplete as a general investigation, but embraces a connected 

 examination of a great district of the higher and central Alps, calculated to elu- 

 cidate a question which I had particularly proposed to myself, as to the supposed 

 diminution of magnetism with height. It likewise includes some observations as 

 to the influence of extinct volcanos on the Rhine. The second series was made 

 in the P3rrenees in 1835, with almost an exclusive view to the influence of height, 

 and is confined to one small district. One other important point gained was a 

 very accurate determination of the comparative horizontal intensities at Edin- 

 burgh and Paris. The choice of the stations was regulated very much by the 

 views just mentioned : the particular spots of observation, together with the geo- 

 graphical position and elevation of the place, will be given in Table VII. I have 

 thought it better to record in the first place in separate Tables for the two needles, 

 the <ietails of the observations in the order in which they were made, the data 



