Fig. 73.

NOTE 218, p. 316.-Fig. 73 represents a helix or coil of copper wire, terminated by two cups containing a little quicksilver. When the positive wire of a Voltaic battery is immersed in the cup p, and the negative wire in the cup n, the circuit is completed. The quicksilver insures the connection between the battery and the helix, by conveying the electricity from the one to the other. While the electricity flows through the helix, the magnet SN remains suspended within it, but falls down the moment it ceases. The magnet always turns its south pole S toward P the positive wire of the battery, and its north pole toward the negative wire.

NOTE 219, p. 319.-A copper wire coiled in the form represented in fig. 73, is an electro-dynamic cylinder. When its extremities P and n are connected with the positive and negative poles of a Voltaic battery, it becomes a perfect magnet during the time that a current of electricity is flowing through it, P and n being its north and south poles. There are a variety of forms of this apparatus.

NOTE 220, p. 339.-In fig. 74 the hyperbola HPY, the parabola p PR, and the ellipse A EPL, have the same focal distance SP, and coincide through a small space on each side of the perihelion P; and as a comet is only visible when near P, it is difficult to ascertain which of the three curves it moves in.

H

Fig. 74.

P

Y

NOTE 221, p. 343.-In fig. 75, EA represents the orbit of Halley's comet, ET the orbit of the earth, and S the sun. The proportions are very nearly exact.

Fig. 75.

NOTE 222, p. 360.-Fig. 74 represents the curves in question. It is evident that for the same focal distance SP, there can be but one circle and one parabola p PR, but that there may be an infinity of ellipses be

tween the circle and the parabola, and an infinity of hyperbolas H PY exterior to the parabola p P R.

NOTE 223, p. 371.-Let A B, fig. 26, be the diameter of the earth's orbit, and suppose a star to be seen in the direction A S' from the earth when at A. Six months afterward, the earth having moved through half of its orbit, would arrive at B, and then the star would appear in the direction BS', if the diameter A B, as seen from S', had any sensible magnitude. But AB, which is 190,000,000 of miles, does not appear to be greater than the thickness of a spider's thread, as seen from 61 Cygni, supposed to be the nearest of the fixed stars.

NOTE 224, p. 373.-The mass is found in the manner explained in Note 133; but the method of computing the distance of the star may be made more clear by what follows. Though the orbit of the satellite star is really and apparently elliptical, let it be represented by CD O, fig. 14, for the sake of illustration, the earth being in d. It is clear that, when the star moves through C D O, its light will take longer in coming to the earth from O than from C, by the whole time it employs in passing through O C, the breadth of its orbit. When that time is known by observation, reduced to seconds, and multiplied by 190,000, which is the number of miles light darts through in a second, the product will be the breadth of the orbit in miles. From this the dimensions of the ellipse will be obtained by the aid of observation, the length and position of any diameter, as Sp, may be found; and as all the angles of the triangle d Sp can be determined by observation, the distance of the star from the earth may be computed.

NOTE 225, p. 376.-One of the globular clusters mentioned in the text is represented in fig. 1, plate 5. The stars are gradually condensed toward the center, where they run together into a blaze somewhat like a snowball. The more condensed part is projected on a ground of irregularly-scattered stars, which fills the whole field of the telescope. There are few stars in the neighborhood of this cluster.

NOTE 226, p. 378.-Fig. 2, plate 5, represents one of those enormous rings in its oblique position. It has a dark space in the center, with a small star at each extremity.

NOTE 227, p. 378.-Fig. 3, plate 5, may convey some idea of the ring in the constellation of the Lyre mentioned in the text.

NOTE 228, p. 378.-This most wonderful object has the appearance of fig. 4, plate 5. The southern head is denser than the northern. The light of this object is perfectly milky. There are one or two stars in it. NOTE 229, p. 378.-Fig. 5, plate 5, represents this brother system. NOTE 230, p. 379.-Fig. 6, plate 5, represents one of the spindle-shaped nebulæ.

NOTE 231, p. 385.-Elongation. The apparent angular distance of an object from the center of the sun. 0 0

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