PARALLELISM

, the quality of a parallel, or that which denominates it such. Or it is that by which two things, as lines, rays, or the like, become equidistant from one another.

Parallelism of the Earth's Axis, is that invariable situation of the axis, in the progress of the earth through the annual orbit, by which it always keeps parallel to itself; so that if a line be drawn parallel to its axis,| while in any one position; the axis, in all other positions or parts of the orbit, will always be parallel to the same line.

In consequence of this Parallelism, the axis of the earth points always, as to sense, to the same place or point in the heavens, viz to the poles. Because, though really the axis, in the annual motion, describes the surface of a cylinder, whose base is the circle of the earth's annual orbit, yet this whole circle is but as a point in comparison with the distance of the fixed stars; and therefore all the sides of the cylinder seem to tend to the same point, which is the celestial pole.—To this Parallelism is owing the change and variety of seasons, with the inequality of days and nights.

This Parallelism is the necessary consequence of the earth's double motion; the one round the sun, the other round its own axis. Nor is there any necessity to imagine a third motion, as some have done, to account for this Parallelism.

Parallelism of Rows of Trees. The eye placed at the end of an alley bounded by two rows of trees, planted in parallel lines, never sees them parallel, but always inclining to each other, towards the farther end.

Hence mathematicians have taken occasion to enquire, in what lines the trees must be disposed, to correct this effect of the perspective, and make the rows still appear parallel. And, to produce this effect, it is evident that the unequal intervals of any two opposite or corresponding trees may be seen under equal visual angles.

For this purpose, M. Fabry, Tacquet, and Varignon observe, that the rows must be opposite semi-hyperbolas. See the Mem. Acad. Sciences, an. 1717.

But notwithstanding the ingenuity of their speculations, it has been proved by D'Alembert, and Bouguer, that to produce the effect proposed, the trees are to be ranged merely in two diverging right lines.