22 



SOLAR-DIURNAL VARIATION 



the diurnal minima and maxima, the former represented by a valley, the latter by 

 a ridge in the magnetic surface. 



The magnitude of the diurnal range is next to be considered. 



Diagram showing the Deflection (in minutes op arc) op the Noeth end of the Mac.net from its Monthly Normal 

 Position for every hour of the day and month of the year, derived from the Declinometer Observations 



at Philadelphia between 1840 and 1S45. 



January, 



February, 



March, 



April, 



May, 



June, 



July, 



August, 



September, 



October, 



November, 



December, 



January, 



no I i 2-5 | 



■ / /// \\\ 

 ■• ///.-. \\\ 



-v-H 



I ' ' / S"\ I II 



!///J 



I'll /.ill l' 

 I I I I I M l|| I 



I ! ' ', / fT-H try 



'III 'I ' 1 1 



i I <"\IM i 



i l/*^\ 

 I - AV\Ul 



! /! * 



i < n 



ii 



5-5 i j f S ! 



" ; i a ii/ i 



I /' '/ 



i >• =r i 1 1 i ' i i f \ i i i '/ i'i • 

 — - // i WAV' ///#' 



io-oj 



0',5 



J-IO 



J»*' 



/ , — \ 

 ' /2-o\ 



Oli-l 234567 8 9 10 11 N'n 13 14 15 16 17 18 19 20 21 22 23 24>'- 

 Philadelphia meau time. 



The following table contains the amount of the deflection at the eastern and 

 western elongations and the diurnal amplitude of the declination for each month 

 of the year, derived from the preceding equations: — 



The diurnal range for the summer months is 10'.45, for the winter months 5'.56, 



