SECTION XX.. 818 tion, how to find the latitude of a place by the meridian altitude of a star on the south meridian ; but the methods of performance on the north meridian aredifferent. The first way is this. Take the altitude of it when it is upon the north meridian at 5 or 6 or 7 o'clock in the winter, then 12 hours afterwards take its altitude again, for it will be on the me- ridian on the other side of the pole ; substract half the difference of those two altitudes from the greatest altitude, and the re- mainder is the true elevation of the pole, or latitude of the place. The second way. Observe when the star Alioth comes to the meridian under thepole ; then take the height of the pole star, and out of it substract 21 degrees (which is the distance of the pole star from the pole) the remainder will be the true elevation of the pole, or the latitude. The reason of this operation is evident by the xxxth figure, for Alioth is on the meridian under the polejust when the polestar is on the meridian above the pole. Note, The pole star is upon the meridian above the pole just at 12 o'clock at night on the 4th day of May, and under the meridian on the5th day of November; fifteen days after that it will be upon the meridian at 11 o'clock ; thirty days after at 10 o'clock ; so that every month it differs about two hours. Problem XXXIII. " To find the hour of the night by the stars which are on the meridian." If you have a meridian line drawn, and such a board as I have described under the 6th Use of the meridian line, you may exactly find when a star is on the meridian ; anti if you are well acquainted with the stars, wheresoever you 'set up that board up- right on a meridian line, you will see what star is on the meri- dian. Suppose Aldebaran or the bull's eyeon the 20th of January is on the south part of the meridian ; then in some tables find the sun's and that star's right ascension, add the complement of the right ascension of the sun for that day, viz. 3 hours 6 minutes to the right ascension of the star 4 hours 17 minutes, and it makes 7 hours 23 minutes the true hour of the afternoon. Note, If the star be on the north part of the meridian, or below the north pole, it is just the same practice as on the south ; for when any star is on the meridian, the difference between the sun's R. A. and that star's a. A. is the sun's true hour, i. e. its distance from 12 o'clock at noon or midnight, at which time the sun is on the meridian either south or north. If you have no meridian line drawn, you may find within two or three degrees what stars are on the north meridian thus; hold up a string and plummet and project it with your eye over-right the pole star, or rather the pole point, and observe what other stars are covered by itor close to it, for theses are on or near the meridian.
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