The semi-(diurnal/nocturnal) arch being doubled, gives the time of continuance (above/below) the horizon. Or the time of continuance above the horizon, may be found by counting the number of hours contained in the upper part of the horary circle, betwixt the place where the index pointed when the given point was in the Eastern or Western parts of the horizon. If the given point was the Sun’s place, the index pointed the time of his rising and setting, when the said place was in the Eastern and Western parts of the horizon, as in _Prob. 18_. Or the time of Sun-rising may be found by adding or subtracting his ascensional difference, to or from the hour of six, according as the latitude and declination are either contrary or the same way. Thus, at _London_, on the 31st of _May_, the _Sun’s_ _Amplitude_ is 24 degrees Northerly. _Oblique Ascension_, 20. _Oblique Descension_, 58. _Ascensional Difference_, 19. _Semi-diurnal Arch_, 109. His continuance above the horizon, 14½ hours. Sun rises at three quarters past four. Sun sets a quarter past seven. These things for the Sun vary every day; but for a Fixed Star the day of the month need not be given, for they are the same all the year round. In the latitude of 51½ North, _Syrus_’s _Amplitude_ is about 28 degrees Southerly. _Oblique Ascension_, 121. _Oblique Descension_, 75. _Ascensional Difference_, 23. _Semi-diurnal Arch_, 67. Continuance above the horizon, 9 hours. PROB. XXXVIII. _The Latitude and the Day of the Month being given; to find the Hour when any known Star will be upon the meridian, and also the time of its rising and setting._ Having rectified the globe for the latitude of the Sun’s place, bring the given Star to the meridian, and also to the East or West side of the horizon, and the index will shew accordingly when the Star _culminates_, or the time of the _rising_ or _setting_. Thus at _London_, on the 21st of _January_, _Syrius_ will be upon the meridian, at a quarter past ten in the evening; rises at 5¼ hours, and sets at three quarters past two in the morning. By the converse of this problem, knowing the time when any Star is upon the meridian, you may easily find the Sun’s place. Thus, bring the given Star to the meridian, and set the index to the given hour; then turn the globe ’till the index points to 12 at noon, and the meridian will cut the Sun’s place in the ecliptic. Thus when _Syrius_ comes to the meridian at 10½ hours after noon, the Sun’s place will be ≈ ¼ deg. PROB. XXXIX. _To find at what time of the Year a given Star will be upon the Meridian, at a given Hour of the Night._ Bring the Star to the meridian, and set the index to the given hour, then turn he globe ’till the index points to 12 at noon, and the meridian will cut the ecliptic in the Sun’s place; whence the day of the month may be easily found in the kalendar upon the horizon.