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"The theory of the air seemeth at present," he says, "to be perfectly well understood, and the differing densities thereof at all altitudes; for supposing the same air to occupy spaces reciprocally proportional to the quantity of the superior or incumbent air, I have elsewhere proved that at forty miles high the air is rarer than at the surface of the earth at three thousand times; and that the utmost height of the atmosphere, which reflects light in the Crepusculum, is not fully forty-five miles, notwithstanding which 'tis still manifest that some sort of vapors, and those in no small quantity, arise nearly to that height.
By the microscopic examination of a polished surface or of one indented by a reagent, by the determination of the electromotive force of elements of which an alloy forms one of the poles, and by the measurement of the resistivities, the densities, and the differences of potential or contact, the most valuable indications as to their constitution are obtained.
Then I shall explain the phenomena of those rays which are said to suffer refraction on passing through transparent bodies of different sorts; and in this part I shall also explain the effects of the refraction of the air by the different densities of the Atmosphere. Thereafter I shall examine the causes of the strange refraction of a certain kind of Crystal which is brought from Iceland.
Grades may be leveled, additional tracks laid, curves straightened, passenger and freight densities may differ from year to year and from day to day. The attempt to determine the proper rates for each different condition and to change them as conditions change, the employees assert, is obviously absurd.
Torricelli had proved atmospheric pressure, "by showing that this pressure sustained different liquids at heights inversely proportional to their densities;" and Pascal "established the necessary diminution of this pressure at increasing heights in the atmosphere:" discoveries which in part reduced this branch of science to a quantitative form.
If we must compare the eye to an optical instrument, we ought in imagination to take a thick layer of transparent tissue, with a nerve sensitive to light beneath, and then suppose every part of this layer to be continually changing slowly in density, so as to separate into layers of different densities and thicknesses, placed at different distances from each other, and with the surfaces of each layer slowly changing in form.
At sea, where between waves or winds or paddles or screws or machinery, everything is tremor, quiver or jar? In the air, where the balloon is incessantly twirling, oscillating, on account of the ever varying strata of different densities, and even occasionally threatening to spill you out?
The most liberal and loose arrangement gives to each soldier eighty square feet of ground, the next gives forty-two, and the most compact allows twenty-seven feet, without and within his tent. These are densities of population equal to having 348,000, 664,000, and 1,008,829 people on a square mile.
In a similar manner two gases, especially if of different densities, may mingle even when separated from each other by a membrane. In a general way this explains the respiratory changes that occur in the blood in the lungs. Blood containing oxygen and carbon dioxid is flowing in countless tiny streams through the walls of the air cells of the lungs.
A mist was floating white against the dark densities of the woods. He heard the water splashing from the eaves heavily into the gullies below, and then the constable once more raucously cleared his throat. "Thar's a man," he drawled, "a stranger hyarabouts, killed yestiddy in the bridle-path. The cor'ner hev kem, an' he 'lows ye know suthin' 'bout'n it, Constant, 'bout'n the killin' of him.
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