. Éléments d'astronomie théorique et descriptive, pour l'utilisation des collèges et académies. , Méthode amethod très couramment utilisée par l'United States Coast Survey.L'instrument utilisé est le Télescope Zénith, un modificationof l'altitude et l'azimut d'instrument. Les deux étoiles sont soselected que la différence de leurs distances zenith est moins thanthe large champ du télescope. L'instrument est setin le plan du méridien à la moyenne des deux zénith dis-tances, et pour l'étoile qui se termine en premier. Lorsque cette starcrosses le méridien, elle est traversée par le fil de micromètre

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. Elements of theoretical and descriptive astronomy, for the use of colleges and academies. Method, amethod very commonly used by the United States Coast Survey.The instrument employed is the zenith telescope, a modificationof the altitude and azimuth instrument. The two stars are soselected that the difference of their zenith distances is less thanthe breadth of the field of the telescope. The instrument is setin the plane of the meridian to the mean of the two zenith dis-tances, and for the star which culminates first. When this starcrosses the meridian, it is bisected by the micrometer wire, andthe micrometer is read. The instrument is then turned 180° inazimuth, and the process is repeated with the second star. Thedifference of the zenith distances is then obtained from the dif-ference of the two micrometer readings, and added to the halfsum of the two declinations, according to the formula. 78. Fourth Method.—When the local time (either solar orsidereal) is known, the latitude may be obtained from altitudeswhich are not measured on the meridian. Let Fig. 30 be a pro. jection of the celestial sphere on the planeof the horizon. Z is the zenith of the place, P the elevated pole, PZ the co-latitude, andS a star, whose altitude is measured. SPZis the hour angle of the star, which can beobtained from the local time noted at theinstant the altitude is observed. PS is the ^8- w- stars known polar distance. In the triangle UEDUCTIOX OF THE LATITUDE. 6PZ, we have the sides ZS and SP, and the angle SPZ, and cantherefore compute the value of the co-latitude, PZ, by the for-mulae of Spherical Trigonometry. An analytical investigation of the formulae by which this pro-blem is solved shows that errors in the observed altitude and thetime have the less effect upon the result the nearer the body isto the meridian. 79. Methods of Finding the Latitude at Sea.—The second and*he fourth of the methods above described are the methods mostcommonly employed in finding the latitude

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