In what follows of the history of navigation we shall briefly trace the progress of the art, so far as its imptove ment is to be ascribed to the advantages it has derived from the modern discoveries in astronomy, and the in vention of instruments of a suitable construction for mak ing observations on the heavenly bodies at sea.
The methods by which a ship's place is determined on the sut face of the ocean, may be divided into geogra Phical and astronomical. The former, consisting in an application of the principles employed for calculating the ship's position by means of the course and distance only ; and the latter, comprehending thc various problems of thc sphere by which the latitude and longitude of a place may be ascertained from celestial observations. These two general methods of finding a ship's place, though depending on very different data, Friust still be consider ed as having a certain mutual relation, without which the results of the one could not be connected with the deductions of the others. Thus the magnitude of the earth must be known by the measurement of lines on its surface, before celestial observations can, in many cases, be rendered subservient to geographical purposes; and, on the other hand, these very measurements cannot be conducted with a due attention to accuracy, without the aid of tstronomical science. In like manner, the mariner's compass, from the variations to which the needle is liable, ceases to be on all occasions a faithful index of the ship's course, unless its bearings be rectified from time to time, by observations of the heavenly bodies, the only sure guides to which the mariner must ultimately appeal in all his difficulties; and even the log-line, the simplest of all nautical instruments, cannot be constructed of a proper length until the exact magnitude of the earth is deter mined.
The actual magnitude of the earth remained unknown long after its spherical form had been ascertained and ad mitted ; and, indeed, it is only in modern times, when, by means of instruments of a refined and delicate construc tion, the most profound speculations of science have been brought to bear with practical effect on the objects to which they are directed, that the real figure and magni tude of the earth have been determined with certainty and precision. Without stopping, therefore, to examine the results of the more ancient measurements, the linear bases of which are too imperfectly known to enable us to reduce thcm with sufficient accuracy to modern mea sures of length, we shall briefly notice the more recent attempts which have been made, to ascertain these im portant points. Norwood's measurement, which Ivas ex
ecuted in 1635, and described very fully in a work pub lished two years afterwards, entitled The Seaman's .Practice, was immediately applied by the author to cor rect the gross errors which prevailed at that period in the divisions of the log-line ; and though it gave the mag nitude of the earth considerably beyond the truth, it was more correct than any measurement which had been at tempted before that time. A degree, between London and York, corresponded by Norwood's measurement to 367,200 English feet. The measurement of a degree of the meridian was undertaken in France by Picard, in 1669, under more favourable circumstances : a basc line of 5663 toises in length was carefully measured between Villejuive and Juvisy, and this line being connected by a series of triangles with the most remote point, the dis tance between the extreme stations was found by calcula tion to be 68.4301 toises, while the difference of latitude was. discovered, by help of a six feet scctor, to be 1° 11' 57". It was inferred froni this measurement, that a degree of the meridian, the tniddle point of IN hich was in latitude 49° 7', was 57,064 Liaises, or 364,900 English feet. After Newton demonstrated that the influence of attraction, combined with the centrifugal force arising from the earth's rotatory niotion, must render the ter restrial sphere somewhat flattened at the poles, and pro tuberant at the equator, an additional motive was present r.d for prosecuting these measurements with greater ac curacy. In different situations, measurements,on a VC1 y extensive scale, were accordingly set on foot by the mcmt enlightened European governments, and conducted in various quarters of thc globe, with an ardour and assiduity suited to the importance of the undertaking. By a com parison of the different results, it has been established that the earth is an oblate spheroid, of which the com pression at the poles is T-11-.-i-x; thc circumference of the elliptic meridian being 24855.84 miles, ancl that of the equator 24896.16 miles. Hence a geographical or nauti cal mile, deduced from the mean length of a melidional degree, is 6075.6 feet. See ASTRONO31Y.