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24
POPULAR ASTRONOMY
the brass meridian. Consider, now, the various conditions necessary. First, every person who is
![](http://upload.wikimedia.org/wikipedia/commons/thumb/e/ed/Popular_astronomy_airy%281881%29_page_24_figure_9.png/450px-Popular_astronomy_airy%281881%29_page_24_figure_9.png)
Fig. 9.
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