Even their passive electric sense was far beyond my possibility in many of its ramifications. For years I had wondered why their couriers into far regions of the sky could without any chart or landmarks find their way back to their island home with such ease. It could not be by means of vision; for they often went flying above the clouds to the antipodes; nor could it be by smell; for that sense was not nearly so much developed as the others. In some of my now more distant flights with Thyriel I discovered that they homed by the electric sense. It had become keen in the measurements of amounts of electricity; and every locality had its own electric possibilities, not to speak of a certain peculiar quality in its electricity which differentiated it from all others. One of the most important branches of their education was the magnetography of the earth and sky. Although I never got beyond a vague perception of differences in the degrees of electricity, it was of some use to me in my flights to have learned the elements of this great descriptive science. I could tell with fair accuracy how high I was above the earth and whether I was drifting away from Limanora or towards it; for the amount of electricity in any region varied within certain definite limits and the conditions governing it were constant for long periods of time; these were, roughly, the metals beneath the surface of the earth, the differences in temperature of the strata of air above, the evaporation and chemical changes on the earth below, and the periodicity of the influence of the sun and the stars. Their electric charts of the sky and air were ever in process of correction, but so slightly and gradually in each region that it was only after long periods that the Limanoran couriers had to revise their magnetographic knowledge; indeed it was their reports after long flights which generally led to the minute corrections of their charts. It was the work of a few minutes only to learn the new modifications, for their charts were exact miniature models of that which they were intended to represent; the learner had only to touch a spring and by the inner mechanism of the globe out would ray to each point of it the electricities that in degree and quality belonged to the region indicated; the member of the electric family who guided him would explain the changes that had occurred since he last consulted the instrument, and his own electric sense would tell him the rest.