The poles of the horizon are the zenith and nadir. The _zenith_ is the point directly over our heads; and the _nadir_, that directly under our feet. The plumb-line (such as is formed by suspending a bullet by a string) is in the axis of the horizon, and consequently directed towards its poles. Every place on the surface of the earth has its own horizon; and the traveller has a new horizon at every step, always extending ninety degrees from him, in all directions. _Vertical circles_ are those which pass through the poles of the horizon, (the zenith and nadir,) perpendicular to it. The _meridian_ is that vertical circle which passes through the north and south points. The _prime vertical_ is that vertical circle which passes through the east and west points. The _altitude_ of a body is its elevation above the horizon, measured on a vertical circle. The _azimuth_ of a body is its distance, measured on the horizon, from the meridian to a vertical circle passing through that body. The _amplitude_ of a body is its distance, on the horizon, from the prime vertical to a vertical circle passing through the body. Azimuth is reckoned ninety degrees from either the north or south point; and amplitude ninety degrees from either the east or west point. Azimuth and amplitude are mutually complements of each other, for one makes up what the other wants of ninety degrees. When a point is _on_ the horizon, it is only necessary to count the number of degrees of the horizon between that point and the meridian, in order to find its azimuth; but if the point is _above_ the horizon, then its azimuth is estimated by passing a vertical circle through it, and reckoning the azimuth from the point where this circle cuts the horizon. The _zenith distance_ of a body is measured on a vertical circle passing through that body. It is the complement of the altitude. The _axis of the earth_ is the diameter on which the earth is conceived to turn in its diurnal revolution. The same line, continued until it meets the starry concave, constitutes the _axis of the celestial sphere_. The _poles of the earth_ are the extremities of the earth's axis: the _poles of the heavens_, the extremities of the celestial axis. The _equator_ is a great circle cutting the axis of the earth at right angles. Hence, the axis of the earth is the axis of the equator, and its poles are the poles of the equator. The intersection of the plane of the equator with the surface of the earth constitutes the _terrestrial_, and its intersection with the concave sphere of the heavens, the _celestial_, equator. The latter, by way of distinction, is sometimes denominated the _equinoctial_.