Page:Popular Astronomy - Airy - 1881.djvu/38

24 the brass meridian. Consider, now, the various conditions necessary. First, every person who is



acquainted with the celestial globe, knows that the brass meridian ought to be perpendicular to the horizon; for securing that condition in the curve described by the transit instrument, the two points AB must be exactly level. In the next place, the brass meridian of a common globe is not what is called a small circle, but it divides the globe into two equal parts. For that purpose it is necessary that