92 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XXVII.) 
above*; and though this deflection would coincide with that produced by the helix 
alone, still it was important to verify its effect. I therefore took a bar magnet 30 inches 
long, and weak in condition, and suspended the needle above it in various parts, so 
as to have the effect of north or south dip to any degree, or no dip at all near the 
middle parts. The effect of absence of air from beneath was also in a certain degree 
represented ; and to make this point more striking, I occasionally put masses of iron 
on and under the middle part of the magnet. The results with the helix were now 
influenced greatly in the amount of the deflection, but not in the direction. When 
the helix affected the direction of the needle, it was according to the above laws. 
2996. In the consideration of natural phenomena, the magnetic axis, and also the 
planes of the magnetic equator and meridian, being circles or planes of no deflection, 
are very important. Changing as they do with every change, either of place or de- 
clination or dip, they require some ready means of illustration, and can hardly be 
comprehended in their effects without a model. I have prepared a globe on which, 
after marking the places of the observatories, I have drawn the magnetic meridians 
of these places as they were last estimated. I have then in another colour drawn for 
each place its magnetic equator, making that a great circle parallel to the equatorial 
plane of the dipping-needle at the place. I have also marked on the globe the mean 
path of the sun for each month, and by the use of adjustible pins to indicate the 
hours before and after noon of any given place, I have the means of ascertaining 
with sufficient accuracy when the sun is in any particular quadrant, or what part of 
the quadrant ; when it passes a neutral line, and what its position in relation to the 
place of observation is, in a manner which no diagrams or figures could supply. I 
have found the globe very useful ; and I accustom myself to place it always in a cer- 
tain position, namely, with the axis of rotation horizontal, the north pole to my right 
hand, and the astronomical meridian of the [)lace of observation towards the zenith. 
The observer can then regard it as from the place of the rising sun. 
2997. Though we thus have the experimental conditions of a needle under an 
action like that resulting in nature from the presence of the sun (2920.), I do not 
pretend that they can be applied without modification to natural phenomena, but 
only that they give very important aid in the study of the latter and the rationale of 
their action. The atmosphere, instead of being illimitable, wraps the earth round 
as a garment; the influence, as it extends from the region of action, must, in respect 
of that portion which is conveyed through its mass (2920.), curve with its curvature, 
and give a result in any particular place, which only refined calculations founded 
upon careful observations can determine accurately. In regard to the development of 
the air action, it would, I think, be very interesting to ascertain, even roughly, the 
* Referring to the typical globe of cold air (2874.), it is manifest that if the space below the horizontal 
lines a, c, he. were occupied by matter holding the lines in it, then the deflections now represented on the lower 
parts would appear above the holding surfaces, and to a much greater degree, though extending downwards 
to a much smaller space. 
