TERRESTRIAL MAGNETIC INTENSITY. 



17 



Table VII. — {continued). 



Place. 



Particular Situation. 



Latitude, 



N. 



Long, 

 from Paris. 



Height, 

 Eng. feet. 



Observed intensity; Paris = 1.600. 



Needle, No. I. 



Mean. 



Flat Needle. 



Mean. 



Luz, Hautes Pyrenees, 



Luz, Hautes Pyre'nees, 



Gavarnie, .... 

 Ste Marie, Valle'e de\ 

 Campan, . . J 

 Pic duMidi de Bigorre, 



Breche de Roland, 



Field E. of the village, 20tli July, . 



28th . 



Near the road, to the N. of the inn. 

 In a field above the village, . . . 

 Summit, 



W. side ; N. aspect of the Breche, . 



42 51 



42 61 



42 43 

 42 69 

 42 65 



42 41 



2°20W 



2 20 



2 21 

 2 07 

 2 14 



2 20 



2400 ■{ 



2400 



4500 

 2800 

 9600 ( 



9300 



1.123 1.126 

 1.122 1.131 



1.126 1.124 



1.127 1.126 



1.121 1.120 



1.122 1.121 

 1.121 

 1.122 1.123 



} 



1.125 



1.126 

 1.120 



}'■ 



121 

 1.123 



1.124 1.123 



1.123 



1.124 



1.121 1.120 



1.117 1.116 

 1.114? 



1.123 

 1.124 

 1.120 



1.116 



^ 4. On the Direction of the Isodynamic Lines (for horizontal Intensity) in the 

 Central Alps, and in the Pyrenees, and on the Influence (^Height. 



25. The next question comas to be how to deduce the general results con- 

 tained in the preceding tables. Where it is merely required to deduce the posi- 

 tion of Isodynamic Lines (which may be considered as sensibly straight for a dis- 

 trict of moderate extent), projection of the results upon paper would afford quite 

 a sufficient approximation, where the stations are sufficiently multiplied. Thus 

 the variations in latitude and longitude would be determined, and lines of inten- 

 sity 1.00, 1.01, 1.02, &c. might be drawn with great accuracy upon a geographi- 

 cal map. 



26. But the same process will not suffice, if we have a third variable, such 

 as height, and require to extract its influence. The problem, then, is not to draw 

 lines, but planes of equal intensity. For its solution I resolved to use the method 

 of least squares,* which is peculiarly applicable to a question of the kind just 

 stated, and may be made to give, as will immediately be seen, the most probable 

 value of the four following quantities, viz. the variation of intensity for 1' of lati- 

 tude ; its variation for V of longitude ; its variation for 100 feet of elevation ; 

 its most probable absolute value at the origin of the co-ordinates, or the station to 

 which the others are referred. 



27. I assumed that the intensity of any point whose co-ordinates of latitude, 

 longitude, and height, might be denoted with sufficient accuracy by an expression 

 of the form 



ax-{-by-\-cz := I (1) 



* It would be absurd to claim any merit For the application of a method so universally known. 

 But lest I should be supposed to have borrowed without acknowledgment the method of reduction em- 

 ployed by Professor Lloyd and Captain Sabine in their excellent Magnetic Survey of Ireland (Fifth 

 Report of the British Association), I desire to state, that I had some years ago proposed to myself the 

 present method of reduction as the only one adapted finally to solve (within the present limits of error) 

 the question of the influence of height, which so greatly complicates the problem. 

 VOL. XIV. PART I. C 



