## Manual of Spherical and Practical Astronomy, Volume 2 |

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adjustment alidade altitude angular distance apparent assumed azimuth centre chronometer circle reading clamp west clock correction coincidence collimation axis computed constant deduced denote determined difference direction division eliminated equal equations of condition equatorial intervals equatorial star eye piece flexure follows formula given gives graduated heliometer Hence horizon glass hour angle inclination latitude lens limb magnifying power mean error measure meridian circle micrometer thread microscopes middle thread moon's motion nadir nearly normal equations number of observations obtained ocular optical parallax parallel perpendicular pivots placed plane pole position circle prime vertical probable error rays reduced refraction reversed revolves right ascension rotation axis screw sextant sidereal sight line spirit level star star's substitute suppose telescope theodolite tion transit instrument transit threads tube unknown quantities vernier vertical circle weight whence zenith distance zenith telescope zero

### Popular passages

Page 95 - ... to the plane of the instrument may be adjusted. At P and Q are colored glasses of different shades, which may be used separately or in combination to protect the eye from the intense light of the sun. In order to observe with accuracy and make the images come precisely in contact, a...

Page 307 - If the distance of the point from the origin, x=a> when v~ 0, c=2fa, and v*=2f(ax) ; (112) and since only the ordinates on the axis of x are involved, the velocity of the point on the curve depends not on the curve described, but on the difference of the ordinates...

Page 486 - ... is not limited to the case of direct observations, but is entirely general. The principle is readily extended to observations of unequal precision. For if the degree of precision of the observations M, M', M".

Page 97 - The threads of the telescope being made parallel to the plane of the sextant, the mark should be seen in the middle between them. The adjustment of the telescope when necessary is effected by means of two small opposing screws in the ring which carries it. • 85. The index correction. — Having made the preceding adjustments, it is necessary to find the point of the graduated arc at which the zero of the vernier falls when the two mirrors are parallel ; for all angles measured by the instrument...

Page 367 - The telescope is eqnatorially mounted ; that is, it has two axes of motion, at right angles to each other, one of which is parallel to the earth's axis ; consequently the telescope may be made to follow the diurnal movement of a heavenly body by revolving it on this latter axis alone, which is commonly called the polar axis, because it is directed toward the pole of the heavens.

Page 92 - If a ray of light suffers two successive reflections in the same plane by two plane mirrors, the angle between the first and last directions of the ray is twice the angle of the mirrors.

Page 96 - Hold the instrument so that its plane shall be nearly vertical, and bring the direct and reflected images of the sea horizon into coincidence. Then incline the instrument until its plane makes but a small angle with the horizon ; if the images still coincide, the two glasses are parallel: consequently, if the index glass is perpendicular to the plane of the sextant, the horizon glass is also in adjustment. Any distant and well defined terrestrial object may be substituted for the star or the sea...

Page 367 - N (the polar axis) is directed toward the elevated pole of the heavens, and it therefore makes an angle with the horizon equal to the latitude of the place (p.

Page 570 - TABLE XIV. Mean Eeduced Refraction for Lunars. (Vol. I. Art. 249.) " XIV. A. Correction of the Mean Eefraction for the Height of the Barometer. (Vol. I. Art. 249.) " XIV. B. Correction of the Mean Eefraction for the Height of the Thermometer. (Vol. I. Art. 249.) " XV. Logarithms of A, B, C, D, for correcting Lunar Distances. (Vol. I. Art. 249.) " XVI. Second Correction of the Lunar Distance. (Vol. I. Art. 249.) " XVII. A and B. For finding the Correction of the Lunar Distance for the Contraction...

Page 126 - The abstract beauty and advantage of this principle seem to be counterbalanced in practice by some unknown cause, which, probably, must be sought for in imperfect clamping.