(William), an excellent mathematician and astronomer, was born at Dublin in 1656, and admitted into the university of that city; which when he left, he carried with him, a testimonial drawn up in an uncommon form, and in the strongest terms, signifying the high opinion conceived of his genius, the probity of his manners, and the remarkable progress he had made in letters. In 1675, he entered in the middle-temple, where he spent three years in the study of the laws of his country: but the bent of his genius lay strongly toward mathematics and philosophical studies; and even at the university he conceived a dislike to scholastic learning, and fell into the methods of Lord Bacon. In 1683, he formed a society in Dublin, for carrying on the same design with the royal society in London. He soon got a few ingenious men to meet at stated times under proper regulations. Their number immediately increased; Sir William Petty was their first president, and Mr Molyneux their first secretary. Their society continued to meet till 1688, when the confusion of the times dispersed them. Mr Molyneux's reputation for learning recommended him, in 1684, to the notice and favour of the first and great duke of Ormond, then lord-lieutenant of Ireland; and chiefly by his grace's influence he was appointed, that year, with Sir William Robinson, surveyor-general of his majesty's buildings and works, and chief engineer. In 1686, he was sent abroad by the government to view the most considerable fortresses in Flanders. He travelled, in company with Lord Mountjoy, through that country, Holland, part of Germany, and France. Upon his return from Paris to London, in April 1680, he published his Scotohericum Telecopium, containing a description of the structure and use of a telecopical dial invented by him. The severities of Tyrconnell's government forced him, with many others, into England, where he spent two years with his family. In this retirement he wrote his Dioptrics, dedicated to the royal society. A parliament being called in Ireland under Lord Sidney in 1692, Mr Molyneux sat in it as one of the representatives of the university of Dublin. Upon the close of the session, the university honoured him with the degree of doctor of laws; and by the lord-lieutenant he was appointed one of the commissioners for the forfeitures in Ireland, with a salary of 500l. per annum. The last favour he entirely declined, as engaging him in an invidious work. Not long before he died, he published "The Case of Ireland stated, in relation to its being bound by Acts of Parliament made in England." Among many persons with whom he maintained correspondence and friendship, Mr Locke was in a particular manner dear to him, as appears from their letters. In 1688, he made a journey to England on purpose to pay a visit to that great man; and not long after his return to Ireland was seized with a fit of the stone, and died in 1698.—His son, Samuel Molyneux Esq; was born in 1689, and became secretary to George II. while prince of Wales, and one of the lords of the admiralty; in which place he died. He was a gentleman of great learning, especially in mathematical and philosophical subjects.