P RE F A СЕ. . IN every commercial state, any work that has for its object the improvement of the art of Navigation, will always be favourably received; and should it not, in all respects, answer the public expectation, the author may still have some claim to indulgence from his good intentions, and having exercised his best abilities for the good of his country. With this view, the author of the following Treatise has bestowed much time and labour to render it as complete, and as generally useful, as possible; and, though he has not the vanity to imagine it to be by any means a perfect work, he has yet been flattered, that it may be of service to Navigation, and, therefore, not altogether unworthy of the attention of the public. He ventures, therefore, to submit it, with much diffidence, to their judgement; and if it shall be found any ways deserving of such a character, he will esteem their approbation his highest and best reward. Should this be the case, he may be encouraged, at a future period, to offer some opinions on the mechanical construction of ships, an ancient art, but perhaps less improved by long experience than any which we now cultivate with so much success. It is well known, that, although the course and distance between any two places, whose latitude and longitude are accurately settled, may be ascertained with the greatest precision, yet a ship at sea is so liable to be put out of the proper course, by storms, contrary winds, currents, &c. that, after all reasonable allowance is made for the errors arising from these causes, the place of the ship by account is very seldom found to agree with its real place, or that deduced from observation, the difference frequently amounting to several degrees, degrees. It is, therefore, obvious, that the ship may be exposed to the greatest hazard when the seamen think themselves most secure; and from this it appears that there is an absolute necessity of taking observations, whenever an opportunity offers, for obtaining, with accuracy, the true place of the ship. At sea, the latitude of a ship is easily deduced either from the meridian altitude of the Sun, or from that of a star or planet, or from double, treble, &c. altitudes of any of these objects; but with regard to the longitude, a more difficult operation becomes necessary. At land, indeed, the longitude of any place may be found with very little trouble by several methods, particularly by the eclipses of Jupiter's satellites; and the longitude at sea might be found by this method with the same facility, provided a telescope possessed of a sufficient magnifying power could be employed for that purpose. But this is found to be extremely difficult, by reason of the perpetual agitation of the sea. However, attempts to render a telescope manageable at sea have been made by Dr. Irwin and others, and an instrument for this purpose is described in Rutledge's "Theorie Astronomique” But a telescope, with a small magnifying power, be used at sea in moderate weather; and, since, even with such an instrument, solar eclipses and occultations may often be observed with tolerable accuracy, and consequently the longitude may be ascertained from such observations, the opportunities of making them ought never to be neglected when the ship is either out of sight of land, or near an unknown coast. The calculations, indeed, which necessarily attend these methods, seem to be considered as too laborious and difficult for the generality of scamen; and, therefore, the method by observing the distance between the Moon and the Sun, or a fixed star, is now commonly preferred, both on account of its superior facility, and because it can be practised at sea almost as often as necessary. Yet, as even this method is attended with a calculation that is, by many navigators, thought troublesome, various ways have been proposed, to shorten, as much as possible, the may * This method is mentioned in the Cosmolale de Jacques Besson, printed at Paris in 1907. operations operations for reducing the apparent to the true distance; and it is now accordingly reduced to a tolerably simple computation. By this method the longitude may, in most cases, be determined within half a degree, which at sea is esteemed no very great error; and if the calculations are accurately performed, from the mean of several sets of good observations, taken at short intervals of time, the error will, probably, be greatly diminished. If a chronometer, or time-keeper, could be constructed, so as to go uniformly when placed in every different position, and under different degrees of heat, then would this method of finding the longitude at sea be a most valuable acquisition to the navigator. Indeed some of our ingenious countrymen have brought this art to a degree of perfection formerly unknown; and every person acquainted with the principles of watch-making must highly admire the accuracy of Harrison's time-keeper. Those made by Messrs. Arnold, Kendal, Earnshaw, &c. are also excellent; but it is to be hoped, that instruments of this kind may be still farther improved, and may be afforded much cheaper than at present: for the high price alone is a very great objection to them, and very much prevents their being more generally used. It is well known, that every chronometer hitherto contrived is subject to irregularities; the smallest shock is found to affect them, and the rate of going is found to be altered by changes in the atmosphere, even though provided with thermometer pieces. Upon this and other accounts, therefore, their accuracy is very much to be suspected; so that at present they are chiefly used for experiments, or to connect observations, for which purpose they certainly make a valuable appendage to a set of nautical instruments. The following work, which contains the method of finding the longitude already mentioned, and also those from which any advantage can be derived for the solution of this important problem, is divided into two volumes; and the first volume is divided into six books. The first book contains the general principles necessary for a proper knowledge of the subject. Book second contains the description, rectification, and use of the Quadrant, Sextant, and Circular Instrument, in their present improved state; also, an account of the corrections to be applied to b the |