Whatever may be the practical belief of the learned, their fundamental principles forbid the avowal of a plenum, although the undulatory theory of light renders a plenum necessary, and is so far virtually recognized by them, and a correction for resistance is applied to the Comet of Encke. Yet there has been no attempt made to reconcile these opposing principles, other than by supposing that the celestial regions are filled with an extremely rare and elastic fluid. That no definite view has been agreed on, is not denied, and Sir John Herschel speculates on the reality of a resisting medium, by suggesting questions that will ultimately have to be considered, as: "What is the law of density of the resisting medium which _surrounds_ the sun? Is it in rest or in motion? If the latter, in what direction does it move?" In these queries he still clings to the idea of Encke, that the resistance is confined to the neighborhood of the sun and planets, like a ponderable fluid. But the most profound analyst the world has ever boasted, speaks less cautiously, (Poisson Rech.) "It is difficult to attribute, as is usually done, the incandescence of aërolites to friction against the molecules of the atmosphere, at an elevation above the earth where the density of the air is almost null. May we not suppose that the electric fluid, in a neutral condition, forms a kind of atmosphere, extending far beyond the mass of our atmosphere, yet _subject to terrestrial attraction_, yet _physically imponderable_, and, consequently, following our globe in its motion?" The incandescence of aërolites must, therefore, be owing to friction against the molecules of the electric fluid which forms an atmosphere around the globe. According to this view, some force keeps it there, yet it is not ponderable. As it is of limited extent, this is not the medium whose undulations brings to light the existence of the stars; neither is Encke's, nor Herschel's, nor any other resisting medium. Where shall we find the present established principles of science? If we grant the Newtonians a plenum, they still cling to attraction of _all matter_ in some shape. If we confine them to a vacuum, they will virtually deny it. Is not this solemn trifling? How much more noble would it be to exhibit a little more tolerance, seeing that they themselves know not what to believe? We do not offer these remarks as argument, but merely as indications of that course of reasoning by which we conclude that the upholders of the present systems of science are not entitled to any other ground than the pure Newtonian basis of an interplanetary vacuum. DIFFICULTIES OF THIS VIEW.