March 7, 1955
Dear Professor Einstein:
I thank you again for the discussion of the first 8 pages of my letter. Here are the quotations from John Herschel and W. Pickering I have mentioned in our last conversation:
There is beyond any question some profound secret and mystery of nature concerned in the phenomenon of their tails; enormous sweep which it [the tail] makes round the sun in perihelion, in the manner of a straight and rigid rod, is in defiance of the law of gravitation, nay, even of the recorded laws of motion.
J. Herschel, Outlines of Astronomy, p. 406
What has puzzled astronomers since the time of Newton, is the fact that while all other bodies in the sidereal universe, as far as we are aware, obey the law of gravitation, comets tails are clearly subject to some strong repulsive force, which drives the matter composing them away from the sun with enormously high velocities.
W.H. Pickering, article Comets in Encyclopedia Americana.