The early misconception, as to the arrangement of the solar system, arose chiefly from the circumstance that, from the position of the earth, we see it in section, not in plan. When two armies meet, it is difficult for the one to comprehend the dispositions of the other, and hence the plan, sometimes resorted to, of employing balloons, to enable the one party to look down from above upon the position taken up by the other. The orbit of the comet is, in like manner, usually so situated, that it commands such a view of the solar system; and the sun and its surrounding planets are seen as distinctly as the central body of an army, with all its outlying forces, is seen from the balloon. Comets, however, occasionally move on the same level with the planets, and it is from a comet with such an orbit that the best view can be obtained of them individually. The comet may, in this case, come so close that the planetary orbs may be caught in the sweep of its tail. A planet may, at one time, be seen so large as to cover most of the celestial hemisphere; at another, so minute as to appear but a point of light in the dark concave.
Then, again, let us consider the rate at which the comet travels. This is by no means an equable one. Sometimes it moves so slowly, that a child might keep up with it; at another, it speeds round with lightning velocity. It is like a coach going down a declivity without a drag. It increases its velocity till it comes to the bottom of the hill, and the mo-
