1 70 Professor Foi bes's Experiments on 



§ 3. Observations on Magnetic Intensity. 



24. It now remains to give the observations which have been 

 made, and reduced in the way already detailed. These con- 

 sist 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 pursue 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 elucidate 

 a question which I had particularly proposed to myself, as to 

 the supposed diminution of magnetism with height. It like- 

 wise includes some observations as to the influence of extinct 

 volcanos on the Rhine. The second series was made in the 

 Pyrenees in 1835, with almost an exclusive view to the in- 

 fluence of height, and is confined to one small district. One 

 other important point gained was a very accurate determina- 

 tion of the comparative horizontal intensities at Edinburgh 

 and Paris. The choice of the stations was regulated very 

 much by the views just mentioned : the particular spots of ob- 

 servation, together with the geographical position and eleva- 

 tion 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 details of the observations in the order in 

 which they were made, the data for correction, and the cor- 

 rected numbers. These are contained in Tables V. and VI. 

 I have thought it needless to incur the labour and expense of 

 printing the individual numbers on which the mean results 

 are founded. They are however preserved in a condition 

 adapted for immediate reference*. 



* Many of the numerical calculations contained in the remainder of 

 this paper, have been made by two of my pupils Messrs. Irvine and Edward, 

 under my own inspection and revision. 



