in the Magnetic Forces of the Earth. 



359 



age temperature of this stratum is the same. But we have seen 



ao 



in the course of the present investigation, that the magnetic 

 lion of the stratum subject to daily variation of temperature, is 

 not proportional to the temperature simply, — that it is dependent 

 upon the evaporation of moisture and the deposition of dew. 

 Whatever may be our explanation of the effect of dew and evap- 

 oration, the diurnal variations of the magnetic force of a parti- 

 cle at any place may be regarded with but little probable error, 

 as proportional to the diurnal variations of the horizontal magnetic 

 force there ; and the tendency of these variations will be to give 

 the needle an oscillatory movement corresponding to the contin- 

 ually shifting line traced through the points where that portion of 

 the horizontal force which is due to the action of the daily stra- 

 tum is the same. For the tendency of the magnetic action of 

 this stratum, at any moment, will be to set the needle at right 

 angles to this line ; which may be regarded as identical with the 

 line of equal molecular magnetic force. 



Having presented these theoretical considerations, let us glance 

 at some of the results of observation. Fig. 25 shows the mean 



Curve showing the Mean Diurnal Variations of the Declination, for the year 1844. 





Fi 



or 26 



540 





T 



X 



541 - 







*^ * » 



X 



/i K 







T 



/ \ 







X~J~X"X~3 









jf 



\ I 







l\ I "1 





it +4^£ : 













4 



fct -d 



1 r III 



uu p 



OS jit 



r A 



DI 

 *t9 - 







X^ t - 



1 ^*\ 



w\ - 



- > -*_ -- 





54 



\l f 



1 | 1 L __J 



51 _ 



— v?— - 





556 - 







. 1 Mil 



-- rr : nzit:_ 



0* 12 3 4 5 6 7 8 9 10 11 12 1314 15 16 17 18 19 20 2122 23 0* 



Increase of numbers corresponds to decrease of declination. 



diurnal variations of the declination, for the year 1844. The 

 curves for the different quarters of the year are of the same form, 

 an d agree also with the curve for the year, or very nearly so, in 



. * The observations of declination, (and the same is true of the observations of hor- 

 izontal force and vertical force,) were made about 20m. after the Observatory hours, 

 opt as our inquiries mainly relate to the laws of the variations, this fact may be 

 ^regarded without material error. 



