Reference to the phenomenon of aberration, which Dr. Pickering has discussed adequately in his essay and which I need therefore mention here only by name, indicated that the ether was not dragged along by material bodies over and through which it might pass. It seemed that it must filter through such bodies, presumably via the molecular interstices, without appreciable opposition. Were this not the case, we should be in some doubt as to the possibility of observing the velocity through the ether of material bodies; if the ether adjacent to such bodies is not dragged along or thrown into eddies, but "stands still" while the bodies pass, there seems no imaginable reason for anything other than the complete success of such observations. And of course these are of the utmost importance, the moment we assign to the ether the rôle of absolute reference frame. THE EARTH AND THE ETHER One body in motion with respect to the ether is our earth itself. We do not know in advance in what direction to expect this motion or what magnitude to anticipate that it will have. But one thing is clear.]* [In its motion around the sun, the earth has, at opposite points on its orbit, a difference in velocity with respect to the surrounding medium which is double its orbital velocity with respect to the sun. This difference comes to 37 miles per second. The earth should therefore, at some time in the year, show a velocity equal to or greater than 18 1/2 miles per second, with reference to the universal medium. The famous Michelson-Morley experiment of 1887 was carried out with the expectation of observing this velocity.]267 [The ether, of course, and hence velocities through it, cannot be observed directly. But it acts as the medium for the transmission of light.]* [If the velocity of light through the ether is $C$ and that of the earth through the ether is $v$, then the velocity of light past the earth, so the argument runs, must vary from $C-v$ to $C+v$, according as the light is moving exactly in the same direction as the earth, or in the opposite direction,]182 [or diagonally across the earth's path so as to get the influence only of a part of the earth's motion. This of course assumes that $C$ has always the same value; an assumption that impresses one as inherently probable, and one that is at the same time in accord with ordinary astronomical observation. It is not possible to measure directly the velocity of light (186,330 miles per second, more or less) with sufficient accuracy to give any meaning to the variation in this velocity which might be effected by adding or subtracting that of the earth in its orbit (a mere 18 1/2 miles per second). It is, however, possible to play a trick on the light by sending it back and forth over several paths, and comparing (not measuring absolutely, but merely comparing) with great minuteness the times consumed in these several round trips. A JOURNEY UPSTREAM AND BACK