Until recently, we believe, it has been considered a settled fact, that the atmosphere was limited to the height of about forty-five miles, that being estimated as the limit at which the earth's attraction would be balanced by the expansive force of the particles of air. But in this problem there is an element of complication in the rotation of the atmosphere with the earth on its axis. Near the surface, and for a great distance upward, the air is but a part of the solid globe, or rather an appendage to it, moving with it in all respects like the denser fluid which constitutes the mighty ocean. But there must be a point in the ascent upward, where the centrifugal force of the particles of air, in the diurnal rotation, must over-balance the power of gravitation; and from that limit, the motions of the atmosphere must be subject to a law of a wholly different character--partaking of the nature of planetary revolution, rather than of axial rotation. The latest speculations as to the height of the atmosphere, seem to have reached only this degree of certainty, viz., that it does not extend so far as the orbit of the moon. Otherwise, it is argued, the superior attraction of that body, in its immediate vicinity, would aggregate a considerable quantity of the air about it, which would tend to retard the motions of the satellite in its orbit, and of the earth on its axis; whereas, the revolutions and rotations of both are known to have been uniform for a period as far back as authentic observation extends. But these speculations, however curious and interesting, are of no practical importance. We shall never be able to traverse the air to any great distance above the earth's surface. Independent of mechanical difficulties, two great impediments will forever prevent the realization of any such ambitions aspirations. These are the increase of cold and decrease of pressure in the upper regions of the air, and the deficiency of oxygen in the rarefied element for the support of animal life. It is well known that at the earth's surface, the pressure on all parts of the body, internal and external, by the weight of the superincumbent atmosphere, is no less than 141/2 pounds to every square inch. The structure of the human body is physiologically conformed by nature to this pressure, and it cannot survive with any very great change of this amount, either by increase or diminution. When one descends into the water, the pressure is doubled at about 32 feet of depth. In ascending in the atmosphere, the pressure is diminished much less rapidly, of course, but quite sensibly when the altitude becomes very great. Messrs. Coxwell and Glaisher are said to have ascended in 1862 to a height of seven and a half miles. One of these gentlemen became entirely insensible from cold and want of oxygen, and the other very nearly so, being obliged to open the valve of the balloon with his teeth for want of the use of his hands.