To find the Altitude, or Elevation of the Aequator above the Horizon.
AS it is to be supposed, that they only have the Aequator Direct, or Right, who live under it, and have the Poles of the World at the Horizon. But it inclines, and becomes Oblique to us in such a proportion, as we recede from it, so that if we were distant from it; for Example, 40 Degrees, so much of Obliquity, or Inclination it hath towards the Horizon; but so many De∣grees as it is distant in Obliquity, or Inclination from the Horizon, so much it is said to be Ele∣vated above it.
Now what, and how great this Elevation is at any set time, is the Question at present and it may be commodiously determin'd upon Aequinoctial Days, if at the very Hour of Noon the Instrument being disposed to the Meridian Line, you receive through the little Sights the Rays of the Sun then in the Aequator; for the Dioptra, according to the Line of Direction, will mark out the Degree of that Elevation, by numbring from the side of the Instrument. In brief, It is in effect but the same thing, as the finding the Sun's Me∣ridian Altitude in the time of the Equinox.
Moreover in the time of Summer, or Winter Solstice the way of this Invention is very facil, and obvious; for the Suns Declination being known to be 23 Degrees and an half, it is but subtract∣ing them from the Degrees of the Summer Solstice and the Question is resolved; for Example, On the 22 of June, at the hour of Noon, the Sun be∣ing in the Tropick of Cancer, or in the first Degree, of that Sign, gives in the Instrument 64 Degrees and 30 Minutes of Elevation; from which 23 De∣grees and 30 Minutes being subtracted, there will remain 41 Degrees, the Altitude of the Aequator in the said City.