All the forces of Nature,--heat, light, electricity and gravitation,-- are silent and invisible. We never see them;
we only know that they exist by seeing the effects they produce. In all Nature the wonders of the "seen" are
dwarfed into insignificance when compared with the majesty and glory of the "unseen." The great sun itself
does not supply enough heat and light to sustain animal and vegetable life on the earth. We are dependent for
nearly half of our light and heat upon the stars, and the greater part of this supply of life-giving energy comes
from invisible stars, millions of miles from the earth. In a thousand ways Nature constantly seeks to lead men
to a keener and deeper realization of the power and the wonder of the invisible.