Among the circumstances attached to the comet of Halley, which will attract attention, is the fact of its gradually decreasing brightness. We have seen that on some occasions of its recorded visits at remote periods, it presented an appearance which filled the people with terror. Every one knows how insignificant an object it was on its return in 1835. If it be true that comets thus waste themselves away, new data will be afforded to aid in forming a physical theory for their explanation. On another occasion, I shall show that this comet may be regarded as a feeler which the solar system throws out into space to ascertain if there be any considerable masses of matter occupying the space which immediately surrounds it.

THE ATMOSPHERE.

Atmospheric Air is Material.-Its Color.-Cause of the Blue Sky.-Cause of the Green Sea.-Air has Weight.-Experimental Proofs.-Air has Inertia.-Examples of its Resistance.-It acquires Moving Force.-Examples of its Impact.-Air is Impenetrable.-Experimental Proofs.-Elastic and compressing Forces equal.-Limited Height of the Atmosphere.-Elasticity proportioned to the Density.-Experimental Proofs.-Internal and External Pressure on close Vessels containing Air.

THE ATMOSPHERE.

THE Atmosphere is the thin transparent fluid which surrounds the earth to a considerable height above its surface and which, in virtue of one of its constituent elements, supports animal life by respiration, and is necessary, also, to the due exercise of the vegetable functions. This substance is generally, but erroneously regarded as invisible. That it is not invisible may be proved by turning our view to the firmament: that, in the presence of light, appears a vault of an azure or blue color. This color belongs not to anything which occupies the space in which the stars and other celestial objects are placed, but to the mass of air through which these bodies are seen. It may probably be asked, if the air be an azure-colored body, why is not that which immediately surrounds us perceived to have this azure color, in the same manner as a blue liquid contained in a bottle exhibits its proper hue? The question is easily answered.

There are certain bodies which reflect color so faintly, that when they exist in limited quantities, the portion of colored light which they reflect to the eye is insufficient to produce sensation; that is, to excite in the mind a perception of the color. Almost all semi-transparent bodies are examples of this. Let a champagne glass be filled with sherry, or other wine of that color. At the thickest part, near the top of the glass, the wine will strongly exhibit its peculiar color, but as the glass tapers, and its thickness is diminished, this color will become more faint and, at the lowest point, it will almost disappear, seeming nearly as transparent as water.

Now let a glass tube, of very small bore, be dipped in the same wine, and the finger being applied to the upper end, let it be raised from the liquid, the wine will remain suspended in the tube, and if it be looked at through the tube it will be found to have all the appearance of water and to be colorless. In this case there can be no doubt that the wine in the tube has actually the same color as the liquid of which it originally formed a part, but existing only in a