ATMOSPHERIC MAGNETISM— COLD EFFECT — ST. PETERSBURGH. 
97 
almost disappears. It is over by 1 o’clock*, after which the upper end of the needle 
follows the sun until 9^ having passed its mean position at 5’*. It then stops, after which 
it moves east until le** or 17 ^; then again stops, or nearly so, until 21^ and then the 
sun-swing comes on, carrying it to extreme east. So here there are two very important 
points to explain, namely, why the needle moves eastward after 9^, and why it does 
not travel westward from IS*" to 20^, but, on the contrary, is travelling eastwards or 
standing still : the explanation, according to my view, is as follows : — St. Petersburgh 
is a place in which, from its position, the upper cold, consequent upon the daily with- 
drawal of the sun, would produce a paramagnetic action (2994. 3003.). This action, 
as the sun set, would begin to appear on the east, and I conclude that at 9‘'-lP' the 
cold region coming up from the east, not on the latitude of the sun’s path, which is far 
to the south, but probably near to that of St. Petersburgh itself, is able at 9-1 1 o’clock, 
during which the needle is stationary, to counteract any remaining tendency west- 
ward, and after that to draw the line of force and the needle end eastward until 17 
o’clock, and to hold it there, after which the sun sends it eastward in the great swing. 
That the cold, considering its probable position, may well direct the needle end east- 
ward till 17^ and the sun region not send it westward from 17 '' to 20'' or 21'*, is seen, I 
think, to be a very natural consequence of the probable position of the two regions 
between these hours. For letting the sun (whose place we know) represent the warm 
region at 17 ’^, he is then in the eastern quadrant below the horizon, so that if he could 
affect the needle through or round the earth (2995.), it would be to easten it, and it 
continues in that quadrant until 19''. Then at 19'', when he enters the quadrant, in 
which he begins to exert a westening action on the sun, he is in such a position as 
respects the needle at St. Petersburgh (as is seen by a line drawn over the surface of the 
globe (2996.) and compared with the magnetic meridian and dip), and in so inefficient 
a part of the quadrant (2982.), and also so far off, that it has no power to send the 
needle westward, but only in association with the retreating cold region to hold it 
there, until at 21'', or thereabout, the sun-swing from west to east occurs as in other 
cases. After this the needle follows the sun from 1 o’clock, being, as the hours 
advance, gradually arrested and taken up by the cold region of the next twenty-four 
hours, as already described. 
3011. I have considered the cold eastening as continued until as late as I/'*, which 
would imply probably that until that hour the cold region was east of St. Petersburgh. 
It is very difficult to speak, even in a general manner, of the places or times of things 
so little identified as yet, as the warm and cold regions in the upper atmosphere ; but 
referring to the temperatures on the earth at St. Petersburgh, I may point out, that the 
extreme cold is, in the month of January, as late as 19 and 20 o’clock, and Jive hours 
later than it is in the summer months. I may also point out here, for use in the sum- 
mer months, that the maximum heat varies three hours in the opposite direction ; so 
* The St. Petersburgh observations are at 21^ minutes after each hour ; but I mention the hour without the 
minutes as sufficient for a general statement. 
MDCCCLI. O 
