NOTE 222. p. 399. Fig. 74. represents the curves in question. It is evident that for the same focal distance S P, there can be but one circle and one parabola p P R, but that there may be an infinity of ellipses between the circle and the parabola, and an infinity of hyperbolas H P Y exterior to the parabola p PR.

NOTE 223. p. 411. 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 afterwards, the earth having moved through half of its orbit, would arrive at B, and then the star would appear in the direction B S', if the diameter A B, as seen from S', had any sensible magnitude. But A B, 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. 414. 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 C D O, fig. 14., for the sake of illustration, the earth being in d. It is clear that, when the star moves through CD 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. 416. One of the globular clusters mentioned in the text, is represented in fig. 1., plate 5. The stars are gradually condensed towards the centre, 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 neighbourhood of this cluster.

NOTE 226. p. 419. its oblique position. extremity.

Fig. 2. plate 5. represents one of those enormous rings in
It has a dark space in the centre, with a small star at each

NOTE 227. p. 419. Fig. 3. plate 5. may convey some idea of the ring in the constellation of the Lyre mentioned in the text.

NOTE 228. p. 419. 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. 419. Fig. 5. plate 5. represents this brother system.

NOTE 230. p. 420. Fig. 6. plate 5. represents one of the spindle-shaped nebulæ.

NOTE 231. p. 427. Elongation. The apparent angular distance of an object from the centre of the sun.

Assyrians made use of the week of seven

days, 99.

Astronomical tables, 71.

— data for, 71.

-eras, 100. Note 147.
Astronomy, physical, 3.

of the Chinese and Indians, 102.
Atmosphere, analysis and pressure of,
138.

the law of its density, 139.
the effect of heat on, 139.
the extent of, 141.
oscillations of, 142.

of the moon and planets, 262.
of the sun, 263.

of comets, 392.
Atomic weights, 127.

Attraction of a sphere and spheroid, 5.

of the earth and moon, 6.

of the celestial bodies, 6.

universal, 7.

capillary, 134.

electrical, 302.

magnetic, 339.

of electric currents, 352.

Aurora, 320.

Axes, lunar, 79.

—, major, of planetary orbits inva-

riable, 25. 85.

25.

Barometrical measurements, 140.
Base, trigonometrical, 58. Note 125.
Batsha, tides at, 115.

Battery, Voltaic, 323.

Beckman, M., his discovery of the
chemical rays, 232.

Becquerel, M., his experiments and
opinions of electrical phenomena,
310. His theory of atmospheric elec-
tricity, 312. His formation of crys-
tals, 330. His thermo-electric battery,
364.

Bessel, Professor, his notice of the secu-
lar variation of the ecliptic, 95.
Biela, M., discovers a comet, 386.
Binary systems of stars, 340.
Bissextile, or leap-year, 98.

Biot, M., his ascent in a balloon, 141.
His experiments on sound, 156. On
circular polarization, 220. His theory
of electrical light, 309. Of terrestrial
magnetism, 367. On the disturbances
of terrestrial magnetism, 368. His
observations on the magnetic force
during his aërostatic expedition, 370. .
Birds, their dispersion, 299.
Bonnycastle, Capt., his account of a
luminous appearance in the sea,
318.

connection of, with mean motion, Bonpland, M., his botanical observ-

ations, 294.

Botto, Professor, his experiments on
thermo-electricity, 364. 372.

Bouguer, M., his mensuration of a de-
gree of the meridian at the equator,
59.
Bradley, Dr., his discovery of nutation,
94. His tables of refraction, 178.
He mentions the two stars of y Vir-
ginis, 407.

Brahmins employed the week of seven
days, 99.

Brewster, Sir David, his discovery of
fluids in the cavities of minerals, 120.
His analysis of solar light, 185. His
law of the polarizing angle, 214. His
investigation of the temperature of
springs, 279. His estimate of the
temperature of the poles of maxi-
mum cold, and of the poles of rota-
tion, 288. On the parallelism of the
isothermal and geothermal lines, 289.
His observations on phosphorescence,
318.

Brinkley, Bishop, his value of the mass Coal measures, their early formation,
of the moon, 69.

Brown, Mr., his botany of Australia, Cobalt, a metal, its polarity, 320.

Compass. See Mariner's Compass.

Chaldeans, their observations of eclipses, Compression, 6. Note 11.