Imagine a helix, intersected, in its vertical dimension, by a moving plane.  If necessary to assist the mind, suspend a spiral spring above a pail of water, then raise the pail until the coils, one after another, become immersed.  The spring would represent the helix, and the surface of the water the moving plane.  Concentrating attention upon this surface, you would see a point—­the elliptical cross-section of the wire where it intersected the plane—­moving round and round in a circle.  Next conceive of the wire itself as a lesser helix of many convolutions, and repeat the experiment.  The point of intersection would then continually return upon its own track in a series of minute loops forming those lesser loops, which, moving circle-wise, registered the involvement of the helix in the plane.

It is easy to go on imagining complicated structures of the nature of the spiral, and to suppose also that these structures are distinguishable from each other at every section.  If we think of the intersection of these with the rising surface, as the atoms, or physical units, of a plane universe, we shall have a world of apparent motion, with bodies moving harmoniously amongst one another, each a cross-section of some part of an unchanging and unmoving three-dimensional entity.

Now augment the whole by an additional dimension—­raise everything one space.  The helix of many helices would become four-dimensional, and superficial space would change to solid space:  each tiny circle of intersection would become a sphere of the same diameter, describing, instead of loops, helices.  Here we would be among familiar forms, describing familiar motions:  the forms, for example, of the earth and the moon and of their motion about the sun; of the atom, as we imagine it, the molecule and the cell.  For is not the sphere, or ovoid, the unit form of nature; and is not the spiral vortex its characteristic motion, from that of the nebula in the sky to the electron in the atom?  Thus, on the hypothesis that our space is traversing four-dimensional space, and that the forms of our space are cross-sections of four-dimensional forms, the unity and harmony of nature would be accounted for in a remarkably simple manner.

The above exercise of the imagination is a good preparation for the next demand upon it.  Conceive a dichotomous tree—­one that always divides into two branches—­to pass through a plane.  We should have, as a plane section, a circle of changing size, which would elongate and divide into two circles, each of which would do the same.  This reminds us of the segmentation of cell life observed under the microscope, as though a four-dimensional figure were registering its passage through our space.

THE ELECTRIC CURRENT