58 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XXVI.) 
passes from one extremity of its variation to the other, the north or upper end travel- 
ing in the reverse direction to the sun, so that it and the sun cross the meridian 
together in opposite directions, nearly about or a little before noon. About 2 o’clock 
the needle is arrested, and after that time returns west, following the sun. It will be 
proper to state, that the north end of the needle, the motion of which has just been 
described, is the end towards the equator, and also, the upper end of a dipping- 
needle at Hobarton. This distinction will receive more significance presently. 
2898. Hence the cause which affects the needle appears to be far more powerful, 
and more concentrated in time when the sun is present than when he is away. In 
this there is accordance between the time of the effect and the time when the sun 
could exert most influence on those magnetic conditions of the atmosphere, which 
are for the present supposed to govern that effect. 
2899. It will be seen by examination of fig. 1 0, that the time of maximum temperature 
is not when the sun is on the meridian, but two hours after it, both in summer and 
winter. But in reference to temperature and its effect on the magnetic condition of 
the air, and through that on the needle, it is not the local temperature which is sup- 
posed to influence the needle, but that which affects enormous masses of air, above 
as well as below, and of which the temperature at the spot, however important it 
may become when we can properly interpret it, gives us as yet little or no knowledge. 
Still there are some points on which temperature has a more direct bearing. Thus 
the amount of variation of temperature is in summer double what it is in winter, 
and the amount of variation in the declination increases in the same proportion 
(2890.). The minimum temperature in winter is later than in summer, and the ex- 
treme western declination of the needle is also later at the same period. 
2900. The varying direction of the magnetic lines of the earth is made known to 
us by observations in two planes, one the horizontal plane, to which the position east 
and west is referred, constituting declination, and the other a vertical plane passing 
through the line of mean declination, and supplying observations of inclination. 
The direction of the line of force referred to this plane might change so as either to 
increase or diminish the inclination, and it does increase at some places for the 
same hour of local time for which it diminishes at others ; thus it increases at Green- 
wich whilst it diminishes at St, Helena, which is nearly in the same meridian. At 
Hobarton it changes rapidly at the east and west extremes of the variation, i. e. about 
2 and 21 o’clock. From noon it diminishes until about 3 o’clock ; it then continues 
nearly the same in summer, when the variation is greatest until 18 or 19 o’clock, 
from that time it increases until about 22 o’clock, and is nearly a maximum from 
thence till noon. Hence it will be understood, that the inclination is generally 
greatest during the rapid journey of the north end of the needle from west to east 
between 21 and 2 o’clock, and least in the other or prolonged half of the journey; 
and though this is partly broken up in the night effect, to be considered hereafter, 
still as a general result it always appears. 
