“Tobias Mayer is universally considered as one of the greatest astronomers not only of the eighteenth century, but of all times and of all countries.” This is how the French astronomer, Jean Delambre, in his posthumously published Histoire de l'astronomie au XVIIIième siècle (Paris, 1827), introduces his readers to the Göttingen professor, whom Leonhard Euler had already recognized in 1760 as “undoubtedly the greatest astronomer in Europe”. Delambre's placing of Mayer in a historical perspective assumes special significance on account of the fact that he had previously written three volumes on ancient and medieval astronomy.

The most reliable source for a reconstruction of Galileo's progress toward a science of motion is the series of undated fragmentary notes on that subject preserved in Codex A of the Galilean manuscripts at Florence. A gathering of such fragments was published by Favaro in the National Edition of Galileo's works, following the Discorsi. The more sophisticated fragments are clearly associated with the composition of that work, and show a definite and consistent understanding of acceleration. Eliminating those, it will be found that the earlier notes fall into recognizable groups. First, there are some that refer to “moment of gravity”, or to the impetus of a body along a line of descent, and are associated with the discussion of inclined planes in De motu. Second, some refer to descent along arcs and chords of circles, associated with Galileo's letter of 29 November 1602 to Guido Ubaldo. These first two groups of notes do not explicitly refer to accelerated motion, and should not be assumed to do so implicity, where such an assumption can be avoided.

This paper sets out to examine the changes which took place in Thomas Young's concepts of the ether between 1799 and 1807. During the earlier part of this period he supposed the ether to consist of mutually repelling subtle particles which are attracted to particles of matter. Hence, he considered that the ether is denser within dense bodies than in rare ones. Furthermore, Young proposed that the ether density does not change abruptly at an interface; instead the denser ether extends beyond the geometrical limit of a body to form an atmosphere around it. As this hypothesis of an atmosphere of dense ether surrounding material bodies will be the central subject of this paper, I shall in future refer to it as the ether distribution hypothesis.

In 1864 nine eminent scientists, who had long been intimate friends, formed a dining club in order to prevent their drifting apart due to their various duties, and in order to further the cause of science. The club, which acquired the title of “X Club”, held monthly meetings from October to June, and was extremely active for two decades, but then gradually lessened in vitality. It served as a highly significant fraternity of scientists, and the regular communication which it afforded helped the members to marshall their efforts on behalf of science against what they felt to be the obstructionist activities and ideas of conservative scientists, certain theologians, and non-scientific society figures.

It is a matter of record that Henri Poincaré never responded publicly to Einstein's Special Theory of Relativity (RT). Since almost no private papers of Poincaré are available, his attitude toward Einstein's work and his silence on that score become somewhat of a mystery. It is almost certain that Poincaré knew of Einstein's work in RT. First, he was fluent in German, having learned it as a young man when the Germans occupied his home town of Nancy in 1870. Second, he often reported to the members of the Académie des Sciences on current work in electrodynamics in Germany. It is highly improbable that he would have missed the abstract of Einstein's first paper on RT or the subsequent articles by Einstein on the subject, especially those which were translated into French, since they were in areas directly related to his own interests in theoretical physics.