It is the atmosphere that enables us to account for the periodical changes in the plumage of birds and the furs of animals, and the variety of colours to be found amongst them.  By means also of the elasticity of the atmosphere, sounds and odours are transmitted to sensitive beings.  Atmospherical phenomena, it may be safely inferred, attracted the observation of mankind in the earliest ages:  we know that the Egyptians and the Greeks wrote upon the subject; the Jews too, a pastoral people, “could discern the face of the sky;” and even in our day, shepherds may be ranked among the weather-wise.  “This is a fine morning, a soft day, or a cold evening,” are modes of salutation with us, as commonly as is the “Salem Alikem” (Peace be with you!) amongst the inhabitants of the more serene countries of the East.  Shenstone says, though with nearly equal spleen and truth:  “there is nothing more universally commended than a fine day:  the reason is, that people can commend it without envy.”

Why do we call the atmosphere a fluid?

Because it has a tendency to move in all directions, and consequently rushes in and fills every space not previously occupied by a more solid substance.  Hence we find, that every cave, crevice, place, and vessel, having communication with the atmosphere, if it be not filled with something else, is filled with air; against which it is no argument that we do not see it, as it is perfectly transparent, and consequently invisible.

Why do birds fly?

Because of the inertia of the atmosphere, which gives effect to their wings.  Were it possible for a bird to live without respiration, and in a space void of air, it would no longer have the power of flight.  The plumage of the wings being spread, and acting with a broad surface on the atmosphere beneath them, is resisted by the inertia of the atmosphere, so that the air forms a falcrum, as it were, on which the bird rises, by the leverage of its wings.

Why is air generally considered to be invisible?

Because, though a coloured fluid, and naturally blue, its colour acquires intensity only, or, in other words, becomes visible only, from the depth of the transparent mass.  According to rigid Newtonians, air is transparent, or, rather, invisible; and the azure colour of the atmosphere arises from the greater refrangibility of the blue rays of light.  Other philosophers imagine that the blue tint is inherent in air; that is, that the particles of air have the property of producing a blue colour, in their combination with light.

Why are the most distant objects in a prospect of a blue tinge?

Because their colours are always tinted by the deepening hues of the interjacent atmosphere.  Again, the blending of the atmospheric azure with the colours of the solar rays, produces those compound and sometimes remarkable tints, with which the sky and clouds are emblazoned.  Hence, the mountains appear blue, not because that is their colour, but because it is the colour of the medium through which they are seen.