"In 1783 he published his paper On the Proper Motion of the Solar System which contained the proofs of his surmises of a year before. ...His second paper on the Direction and Velocity of the solar system (1805) is the best example that can possibly be given of his marvellous skill in reaching the heart of a matter, and it may be the one in which his philosophical powers appear in their highest exercise. For sustained reflection and high philosophic thought it is to be ranked with the researches of Newton in the Principia."