96 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XXVII.) 
3007 . I will proceed to apply these views, and the additional knowledge gained by 
experiment, to the localities formerly considered, and to some new ones between the 
tropics, for the purpose of explaining, if I can, the principles of night action ; of re- 
tardation, more or less, of the effects in relation to local time ; of the difference in 
direction of the declination variation in different months, for the same place at the 
same hours, as pointed out by Colonel Sabine ; of the diminution of dip in one place, 
and increase of it at another for the same local time. In doing so, it will be necessary 
to refer continually to that place which may be considered, in respect of the station, 
as the centre of hot or cold action for the time. I will endeavour to use the word 
region for that purpose, meaning thereby not the whole extent of heated or warmed 
air, nor the centre, but the chief place of the altered portion. It is very manifest that 
in some days in March or September, all the air that is east of the meridian at 21'* or 
22’‘ may be considered warm in comparison of that which is then west of the same 
meridian, and that a resultant of action, which shall be the same for all places, can- 
not exist. 
3008. We are to remember that the eastening and the westening of the upper end 
of the needle, of which I always speak, is produced in two ways. The needle travels 
as positively by the withdrawal of a direct cause of action as it does under the imme- 
diate direct action of that cause, but in the contrary direction (2982.). A westening 
may be the result either of the coming up of the sun on the east of the place of ob- 
servation, or of its withdrawal in the west, after he has passed over the meridian and 
produced the great east swing. 
3009. St. Petershurgh has a mean declination of 6° 10' W., and a dip of 70 ° 30' N. ; 
therefore, though the magnetic and astronomical meridians are not very oblique to 
each other, still the sun or warm region reaches the former from 20' to 40' before the 
latter, and hence the time of the great sun-swing, which is from 20 to 1 o’clock, is 
made earlier than it otherwise would be. The magnetic equator of the needle (2977.) 
forms an angle of about 40° with the earth’s equator, and being thus tilted, it disposes 
the two quadrants chiefly concerned in the daily variation (2979.), so, that in the 
St. Petershurgh summer the warmest region is not only far nearer to the needle, but 
passes through the strongest places of action of the quadrants, whereas in winter it 
is further off, and also in much weaker positions. Hence a cause, as I believe, of the 
o-reat difference in the amount of variation of declination, and also in its character : 
o ^ 
in November, December and January, it is from 4'‘47 to 4''65 only, whilst in June it 
is 1 1'‘52*. See the Tables, p. 82, and the Curves, Plate II. 
3010. In December or January, being St. Petershurgh winter, the sun-swing east 
* The eastening and westening of a free dipping-needle are not properly represented by the movements of 
a horizontal needle, inasmuch as at places with different dip the angle is read off on planes differently inclined 
to the dip itself, and in high latitudes the effect is greatly exaggerated. But though different places may not 
1)6 compared without a correction, the variations for the same place as St. Petershurgh are comparable and pro- 
portionate. 
