Chap. r. PHILOSOPHICAL DISCOVERIES. 23*

serences. And it is obvious that the hours of the day, whichare successive in any one place, are co-existent when you takein the whole globe; so that no hour os the day can be assigned,but a meridian can be likewise assigned where it is that hour atthis present time. The sensible horizon of any place is a planeperpendicular to the plumb-line at that place, and tangent tothe earth’s surface there. The rational horizon is a plane thro*the earth’s centre .parallel to this, whose poles are the zenithand nadir , in the same manner as the north and south poles ofthe world are the poles of the equator. The particular phe-nomena of places depend upon the position of their horizonwith respect to the circles of the apparent diurnal motion of thefun and stars. The horizon of a place at the equator pastesthro’ the poles, and divides equally the equator and its parallels.Hence the days and nights are always equal in such places, andeach of the stars performs one half of its revolution above theirhorizon, and the other half under it. The circles of diurnalmotion are all perpendicular to their horizon, and thereforethey are said to be in a right sphere. When the fun moves inthe equator, he rises directly from their horizon to their ze-nith, and then descends directly to their horizon again ; inother cafes, after rising perpendicularly, he Hopes away in hisparallel towards the north or south side of their zenith, accord-ing to the season of the year ; which must be a considerablerelief to them, as the heat must thereby be abated. At thepoles, their horizon coincides with the equator ; so that thenorthern celestial hemisphere must be always in view of thenorthern pole, being above their horizon, while no part of thesouthern hemisphere is visible to them, being always beneath it;The circles of the diurnal motion being parallel to the equator;and consequently to their horizon, the fun and stars appear tothem to move in parallels to their horizon ; the fixed stars ne-ver rife nor set, and the /lin rises at the vernal equinox and lets

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