98 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XXVII.) 
that whilst from the highest to the lowest temperature in the day is only eleven hours 
in summer, it is nineteen hours in winter, as may be seen by the Temperature Table, 
p. 83. As the day comes on, therefore, in January the highest temperature is only five 
hours after the lowest, which accords generally with the assumed cause of the effects 
on the needle*. 
3012. As I am endeavouring to make St. Petersburgh a general case of night action 
for the explanation of corresponding effects at other places, so I may notice that the 
night action must contain a portion of sun effect which combines with that of the 
cold. The action of the sun is known by observation to be very extended ; in the 
case of St. Petersburgh, the sun, when at the southern tropic and on the meridian, 
is between 80° and 90° from the station, and yet we see by the observations and 
curves how large an effect he produces (3009.). Wherever the sun may be, he is by 
his motion causing changes which are felt simultaneously over the whole globe ; and 
at 9 and 10 o’clock he is in an effectual part of that quadrant which would send the 
needle eastward if the earth were replaced by air, and in the representative experi- 
ments with a helix (2995.) does so send it eastward when a magnet is interposed. 
The night action ought therefore to be greatest in winter, as it is, because the cold 
is then most intense, and also because the action of the distant sun coincides with it. 
It is very probable that many of the curious contortions of the night action which 
appear in the curves of Hobarton, Toronto and elsewhere, may depend upon the 
manner in which, at different hours, these two causes (probably with others) combine 
together. 
3013. Though the declination varies little or nothing between 17'* and 21*^, no 
westening then appearing (3010.), still I should expect a marked action on the in- 
clination at that time, and conclude that it will be on the increase; but I have not 
been able to obtain a table of the daily variation of inclination. 
3014. In the month of February the same remarks apply; but as the sun is now 
coming from the southern signs and drawing nearer to St. Petersburgh its power is in- 
creasing, and this is shown by making the cold eastening for 15’’, lO** and 17’’ less in 
extent than before by more than half a minute (of a degree), and by absolutely over- 
coming it and making a return westwards between 17 '" and 18’’, before the swing to 
the east comes on. In March the effect is still more striking ; the paramagnetic east- 
ening is arrested at 14’’, and the following diamagnetic westening extends to 20’’ ; then 
follows the swing. In April the westening by the warm region is as early as 13’’ and 
continues to 20’’, being very strong. It is interesting to look at the Table of Tempera- 
tures for these months, even as they are obtained at the earth’s surface. As the months 
^ In relation to the cold of the upper atmosphere and the occurrence of its maximum (at certain levels at 
least), not at midnight but hours after, how often do we in this country see a clear bright night, and then just 
before the sun rises, the formation of a veil of clouds high up, and, upon his appearing, their dissolution and 
passing away ! In these cases the clouds show the time of greatest cold above by their formation, and by their 
dissolution its quick reversion and change into increasing warmth. 
