f a star, in astronomy, an arch of the ecliptic, intercepted between the beginning of Aries, and the point of the ecliptic cut by the star's circle of longitude.

Longitude of a place. See Geography.

In the Philosophical Transactions, No. 1, we have an account of a successful experiment in finding the longitude at sea, made with two pendulum-watches by Major Holmes, in a voyage from the coast of Guinea homewards. This and some other successes encouraged Monsieur Huygens so far, that, after he had improved the structure of these watches, he published an account at large for the shewing how and in what manner these watches are to be used in finding the longitude at sea, with directions for adjusting them and keeping a journal by them; which account the curious reader may see at large in the Philosophical Transactions, No. 47.

The chief objection against pendulum clocks and watches, is the effects that heat and cold have upon the spring and pendulum, which make the spring in watches draw stronger at some times than at other times, and causes the pendulum to lengthen and shorten, according as the weather is hotter or colder; but these effects are so regular, that without doubt they may be accounted for.

But the most ingenious and successful machines for this purpose have been invented by Mr. John Harrison, who, at different times, contrived three different time-pieces for determining the longitude at sea.

The first of Mr. Harrison's machines was tried in May 1736, when it was put on board a man of war; and by its exact measure of time, in its return from Lisbon, corrected an error of almost a degree and a half in the computations of the reckoning of a ship. In 1739, Mr. Harrison finished his second machine, which, from various experiments made upon it, was sufficiently regular and exact for finding the longitude of a ship within the nearest limits proposed by parliament. Upon the success of this, Mr. Harrison, in 1741, undertook still a more advantageous machine, which he finished in 1758, when he applied to the commissioners of longitude for orders to make a trial of that instrument to some part in the West Indies, as directed by the statutes for the discovery of the longitude at sea. In consequence of this application, Mr. Harrison received orders for his son to proceed from Portsmouth to Jamaica, in one of his majesty's ships of war, with his third instrument, in November 1761; and the commissioners having directed that every requisite step and precaution should be taken, for making, with care, the proper experiments, and ascertaining their accuracy, not only going to Jamaica, but in the return, it appears, from the calculations made from the experiments in going to Jamaica, that the difference between the longitude, as found by the time-piece, and calculated by the observations of the transit of mercury in 1743 at Jamaica and London, is five seconds of time, which at Jamaica is little more than a geographical mile.

During the voyage, Mr. Harrison's time-piece corrected the ship's reckoning, which sometimes varied about a degree and a half; and in going from Madeira to Jamaica, it also corrected the errors of the log, and showed the longitude so exactly, that the ship made the island of Deseda, and all the other islands, until they arrived at Jamaica, as foretold by the time-piece.

At the arrival at Jamaica, the observations for finding the time were made by equal altitudes; and the longitude shown by the time-piece, being within 5" of time of the longitude shown by the most accurate observations of mercury in its transit over the sun in the year 1743, and with which all the observations at London and Paris agreeing within 23", amounts to a demonstration, that Mr. Harrison has performed all that is required by the statute of the 12th of queen Anne, to entitle him to the greatest reward mentioned in that act.

In returning from Jamaica, the weather was very tempestuous, so that the time piece was forced to be placed on the counter, to avoid being perpetually exposed to the sea water; there it suffered continual violent agitations, which, though they necessarily retarded its motion, yet did not occasion any such considerable error, as would have made Mr. Harrison's right to the greatest reward questionable, had it depended on this voyage only; for the time-keeper, in its going and return, lost only 1' 54" and ½, which, in the latitude of Portsmouth, amounts to about eighteen geographical miles or minutes of a great circle, whereas the act required only that it should come within the distance of thirty geographical miles or minutes of a great circle.