signs of internal activity. The head
had begun to develop and the tail to elongate, till the comet was for a
time lost sight of. No human eye beheld the wondrous spectacle which it
must have offered on December 8th. Only _four days_ afterwards, however,
it was seen again, and the tail, whose direction was reversed, and which
observe could not possibly be the same tail, its tail had already
lengthened out to the extent of about 90 millions of miles, so that it
must have been shot out with immense force in a direction _from_ the
sun."

The reader will have observed it took from November 10th to December
8th, or 28 days, to fall to the sun for the same distance, and that with
all the velocity it had on November 10th to start with. Herschel sums up
the matter thus: "Beyond a doubt, the widest and most interesting
prospect of future discovery which their study (comets' tails) holds to
us, is, that distinction between gravitating and levitating matter, that
positive and unrefutable demonstration of the existence in nature of a
repulsive force co-extensive with, but enormously more powerful than the
attractive force we call gravity, which the phenomena of their tails
afford."

Thus the philosophic mind of Herschel saw in the existence of cometary
tails, the irrefutable evidence of the existence of a repulsive force,
not of a hypothetical character, but as real as the existence of gravity
itself. Various attempts have been made to define that repulsive force
which was thus demanded, and the same force has been ascribed by
scientists to the repulsion due to heat, to light, and also to
electricity.

Several French scientists have suggested that the repulsive force was
due to the heat of the sun. M. Roche was one of those who stated that
the phenomena of cometary tails was due to the repulsive power of heat,
which found its origin in the heat of the sun. M. Faye, another French
scientist, states that the repulsive force had its origin in the heat of
the sun. By a series of experiments he demonstrated that there was a
repulsive power in all heat waves, which gave his theory that
experimental support that any theory must possess to make it permanent.

Now in Art. 63 it was shown that heat does possess a repulsive power,
but that that power is rather due to the electro-magnetic Aether whose
vibrations produce the heat waves, than to the repulsion of heat; so
that, indirectly, the assumption of both these French scientists, that