It is not very probable that it will be satisfactorily visible in this hemisphere, during its return in the present year it will pass its perihelion the early part of May. : The preceding diagram will afford an idea of the magnitude and figure of the orbit of the Encke comet, compared with that of the earth. The comet moves round the sun in about forty months; the figures round the ellipse indicate the positions of the comet in months, reckoning from the perihelion. A P the major axis. CP the semi-axis major. M R the minor axis. A the aphelion. P the perihelion. The dotted line the line of the nodes. N the ascending node. D the descending node. The order of the figures, point out the nature of the comet's motion, that it is direct; or from P to 1, 2, 3, and so on, round the ellipse; were the course from P to 39, 38, 37, &c. the motion would be called retrograde: the comet of Encke is direct; the comet of Halley is retrograde. This diagram illustrates the slowness of a comet's motion at its aphelion, and its swiftness at its perihelion,—from 39 to P is the curve described in one month; from 12 to A is the curve described in eight months. The circle (having the sun in the centre) represents the orbit of the earth, on which is marked the signs of the ecliptic, by which, and the figures round the orbit of the comet, the relative places of the two bodies may be found at any time from a passage of the perihelion. The O, on the orbit of the earth, points out its position at the time that the comet is in perihelion in the present year. The relative positions of the two bodies in the year 1828, at the time when the comet was most distinctly seen, were -the earth near, and the comet at the 39th month, from which, it is evident that it was then so far removed from the solar rays, and so near the earth, as to be in its most favorable position for observation. The return of this comet, in 1828, had been looked for with more than usual interest, on account of a small diminution in its period, observed between each return. This revived the ancient question of a vacuum or plenum, in which the heavenly bodies move, a point that cannot be determined by the planets, their masses being so dense, and an ether so rare, (if it exist,) that immense ages must roll on before its resistance to their motions would affect the period of their revolutions; and not only their density, but also their uniformity of volume, and circular motions, combine to prevent such a resistance being perceptible. With comets the case is widely different, they being bodies exceedingly rare, and, consequently, most likely to be impeded by a medium, such as an ether. In the last return of this comet, some minute variation was noticed between its computed and observed places, but of a nature not sufficient to warrant a confirmation of the existence of this subtle fluid. It has been calculated that the density of the ether, sufficient to produce the acceleration observed in the mean motion of this comet, must be 360.000.000.000 times less than that of atmospheric air. OCTOBER. SOLAR PHENOMENA. To the North Star, looks the Geographer; The Poet, to the "Happy Isles." A meridian line, or a line, which, if produced, would pass through both the poles of the earth, and the spot where the observer is placed, is the basis of all astronomical observation, and has, from the very earliest ages of the science, been considered of great importance. The following method of drawing a meridian line, is the simplest that can be adopted. On a card, or any other convenient superficies, describe two, three, or more concentric circles, and in the common centre, fix perpendicularly a wire, C B, having a well-defined point; this card should be fixed on a perfectly horizontal plane. When the sun shines in the morning, observe where the shadow of the top of the wire, as CD, touches one of the circles; and in the afternoon, mark where the extremity of the shadow C F, just touches the same circle: then through the centre C, draw the line S N. bisecting the arc D F, and S N will be a meridian line, as required. If the same be done |