ASCENT

, the motion of a body from below tending upwards; or the continual recess of a body from the earth, or from some other centre of force. And it is opposed to descent, or motion downwards.

The Peripatetics attributed the spontaneous ascent of bodies to a principle of levity, inherent in them. But the moderns deny that there is any such thing as spontaneous levity; and they shew, that whatever ascends, does so by virtue of some external impulse or extrusion. Thus it is that smoke, and other rare bodies, ascend in the atmosphere; and oil, light woods, &c, in water: not by any inherent principle of levity; but by the superior gravity, or tendency downwards of the medium in which they ascend and float.

The ascent of light bodies in heavy mediums, is produced after the same manner as the ascent of the lighter scale of a balance. It is not that such scale has an internal principle, by which it immediately tends upwards; but it is impelled upwards by the preponderancy of the other scale; the excess of the weight in the one having the same effect, by augmenting its impetus downwards, as so much real levity in the other: because the tendencies mutually oppose each other, and action and reaction are always equal.—See this farther illustrated under the articles Fluid, and Specific Gravity.

Ascent of Bodies on Inclined Planes. See the doctrine and laws of them under Inclined Plane.

Ascent of Fluids, is particularly understood of their rising above their own level, between the surfaces of nearly contiguous bodies, or in slender capillary glass tubes, or in vessels filled with sand, ashes, or the like porous substances. Which is an effect that takes place as well in vacuo, as in the open air, and in crooked, as well as straight tubes. Indeed some fluids ascend swifter than others, as spirit of wine, and oil of turpentine; and some rise after a different manner from others. The phenomenon, with its causes, &c, in the instance of capillary tubes, will be treated more at large under Capillary Tube.

As to planes: Two smooth polished plates of glass, metal, stone, or other matter, being placed almost contiguous, have the effect of several capillary tubes, and the fluid rises in them accordingly: the like may be said of a vessel filled with sand, &c; the various small interstices of which form, as it were, a kind of capillary tubes. So that the same principle accounts for the appearance in them all. And to the same cause may probably be ascribed the ascent of the sap in vegetables. And on this subject Sir I. Newton says, “If a large pipe of glass be filled with sifted ashes, well pressed together, and one end dipped into stagnant water, the fluid will ascend slowly in the ashes, so as in the space of a week or fortnight, to reach the height of 30 or 40 inches above the stagnant water. This ascent is wholly owing to the action of those particles of the ashes which are upon the surface of the elevated water; those within the water attracting as much downwards as upwards: it follows, that the action of such particles is very strong; though being less dense and close than those of glass, their action is not equal to that of glass, which keeps quicksilver suspended to the height of 60 or 70 inches, and therefore acts with a force which would keep water suspended to the height of above 60 feet. By the same principle, a spunge sucks in water, and the glands in the bodies of animals, according to their several natures and dispositions, imbibe various juices from the blood.” Optics, pa. 367.

Again, if a drop of water, oil, or other fluid, be dropped upon a glass plane, perpendicular to the horizon, so as to stand without breaking, or running off; and another plane touching it at one end, be gradually inclined towards the former, till it touch the drop; then will the drop break and move along towards the touching end of the planes; and it will move the faster in proportion as it proceeds farther, because the distance between the planes is constantly diminishing. And after the same manner, the drop may be brought to any part of the planes, either upward or downward, or sideways, by altering the angle of inclination.

Lastly, if the same perpendicular planes be so placed, as that two of their sides meet, and form a small angle, the other two being only kept apart by the interposition of some thin body; and thus immerged in a fluid, tinged with some colour to render it visible; the fluid will ascend between the planes, and that the highest where the planes are nearest; so as to form a curve line which is found to be a true hyperbola, of which one of the asymptotes is the line of the fluid, the other being a line drawn along the touching sides.

And the physical cause of all these phenomena, is the same power of attraction.

Ascent of Vapour. See Cloud and Vapour.

, in Astronomy, &c. See Ascension.

, are those inhabitants of the globe, who, at certain times of the year, have no shadow. Such are the inhabitants of the torrid zone, who twice a year having the sun at noon in their zenith, have then no shadow. |