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The practical value of this science is best appreciated by the navigator, who sees in the sun and moon his clock, and in the stars and planets the ready means of learning his latitude and longitude. It is one of the first tasks of the midshipman to become familiar with the use of the sextant, by which he works out the problem of ascertaining the exact place of the ship upon the ocean. Navigation is helpless without the assistance of astronomy. Yet it is only the A,B,C of the science that the sailor has any use for; its higher mysteries are away beyond his needs and of no practical profit to him.

Nathaniel Bowditch, of Salem, Mass., in 1802, issued a book entitled “The New American Practical Navigator,” which is still a standard treatise for seamen. His rare acquirements as a mathematician were signally displayed, and in a form that has proved enduring, when, in 1814–17, he translated into English, accompanied with copious notes of his own, the profound work, “Celestial Mechanics,” penned by the gifted La Place in 1799. Although in name a translation of a foreign book with a commentary, it is in many respects an original work. Professor Elias Loomis, who left to Yale University three hundred thousand dollars as an endowment fund to aid in prosecuting astronomical research, said of him, in 1850, “Bowditch has probably done more for the improvement of physical astronomy than all other Americans combined.” Dr. Bowditch published the work in four ponderous quarto volumes wholly at his own private cost. These volumes he did not expose for sale, but generously gave them to such persons as proved to him their ability to appreciate and comprehend them. This outlay impaired the fortunes of his family, but became his own unique monument.

This work remains one of the most profound efforts of mathematical research on record. Bowditch’s accuracy has passed into a proverb. He gave the latitude of all the principal seaports of the world with marked precision; while some of the longitudes are now found to be slightly in error, it is surprising that his determinations of those of Boston and Philadelphia should be exactly the same as those obtained by the best methods in use to-day. But he makes San Francisco and Halifax seven miles too far to the east, and New York eight miles too far west. But we are to remember that for this computation the best available instruments were the chronometers of a century ago, and that lunar observations were made with the old-time sextant.

ZENITH TELESCOPE.

Made for University of Pennsylvania by Warner & Swasey.

As applied to geodesy, astronomy has added a process of ascertaining geographical latitude with marvelous accuracy and speed by the use of the zenith telescope, an instrument devised by Major Talcott in 1835. This instrument can be set in a vertical direction with ease, and be pointed alternately to two stars that cross the meridian at a brief interval of time, the one north and the other south of the zenith. Difficulties that arise from refraction are avoided, and the resulting latitude is quickly computed. This method is largely employed in the surveys of the public lands, as also in establishing the boundary between the United States and British America.