Tangent Index and Day and Night Apparatus

              The Tangent Index and Day and Night Apparatus.
John Haywood, Pkofessok of Mathematics, Otteebein Univer¬ sity, Westbrville, Ohio.    1890.
By the aid of this new Instrument, The phenomena of the day and night corresponding to the apparent diurnal motion of the sun, can be exhibited to a class in all their variations. The continuous equal days and nights at the equator; the varying lengths of day and night in the higher latitudes, at different times of the year; the con¬ tinuous day or night in the polar regions; the six months day and six months night at the pole are distinctly shown. These phe¬ nomena are shown better by the day and night instrument than by the celestial globe, because the small sphere, which in this appar¬ atus, represents the sun, is visible to the class during the entire diurnal motion; whereas in the use of the globe, the teacher very frequently finds that the very thing he wishes the class to see, is hidden behind the globe.
To use the instrument, it is better to place it so that it is adjusted to the meridian, as nearly as possible. When the direction of the cardinal points of the compass are known, this can ordinarily be done with sufficient accuracy, by a very simple inspection. If greater accuracy is desired, a small compass is a convenient aux¬ iliary. The teacher should be careful to explain to the pupils that the wooden horizontal circle represents the real horizon; that wher¬ ever one is on the earth the visible heavens are bounded by a horizon as it is seen at home. Though this horizon is in fact more nearly perfect in some localities than in others; that it is more perfect in a level country than in a mountainous; that it is most perfect on the sea, when the horizon very accurately (save for atmospheric refraction,) divides the heavens into two hemispheres; the upper or visible hemisphere, and the lower or invisible hemis¬ phere. In the use of the instrument the horizon is considered of this perfect kind.