In 1354, having shortly before been made a knight, he was sent into England with the lords of Brittany to treat for the ransom of Charles of Blois, who had been defeated and captured by the English in 1347. When Rennes and Dinan were attacked by the duke of Lancaster in 1356, Du Guesclin fought continuously against the English, and at this time he engaged in a celebrated duel with Sir Thomas Canterbury. He finally forced his way with provisions and reinforcements into Rennes, which he successfully defended till June 1357, when the siege was raised in pursuance of the truce of Bordeaux. For this service he was rewarded with the lordship of Pontorson. Shortly afterwards he passed into the service of France, and greatly distinguished himself at the siege of Melun (1359), being, however, taken prisoner a little later by Sir Robert Knollys. In 1360, 1361 and 1362 he was continually in the field, being again made prisoner in 1360. In 1364 he married, but was soon again in the field, this time against the king of Navarre. In May 1364 he won an important victory over the Navarrese at Cocherel, and took the famous Captal de Buch prisoner. He had previously been made lord of La Roche-Tesson (1361) and chamberlain (1364); he was now made count of Longueville and lieutenant of Normandy. Shortly afterwards, in aiding Charles of Blois, Du Guesclin was taken prisoner by Sir John Chandos at the battle of Auray, in which Charles was killed. The close of the general war, however, had released great numbers of mercenaries (the great companies) from control, and, as they began to play the part of brigands in France, it was necessary to get rid of them. Du Guesclin was ransomed for 100,000 crowns, and was charged to lead them out of France. He marched with them into Spain, supported Henry of Trastamara against Pedro the Cruel, set the former upon the throne of Castile (1366), and was made constable of Castile and count of Trastamara. In the following year he was defeated and captured by the Black Prince, ally of Pedro, at Navarette, but was soon released for a heavy ransom. Once more he fought for Henry, won the battle of Montiel (1369), reinstated him on the throne, and was created duke of Molinas.
In May 1370, at the command of Charles V., who named him constable of France, he returned to France. War had just been declared against England, and Du Guesclin was called to take part in it. For nearly ten years he was engaged in fighting against the English in the south and the west of France, recovering from them the provinces of Poitou, Guienne and Auvergne, and thus powerfully contributing to the establishment of a united France. In 1373, when the duke of Brittany sought English aid against a threatened invasion by Charles V., Du Guesclin was sent at the head of a powerful army to seize the duchy, which he did; and two years later he frustrated the attempt of the duke with an English army to recover it. Finding in 1379 that the king entertained suspicions of his fidelity to him, he resolved to give up his constable’s sword and retire to Spain. His resolution was at first proof against remonstrance; but ultimately he received back the sword, and continued in the service of France. In 1380 he was sent into Languedoc to suppress disturbances and brigandage, provoked by the harsh government of the duke of Anjou. His first act was to lay siege to the fortress of Châteauneuf-Randon, but on the eve of its surrender the constable died on the 13th of July 1380. His remains were interred, by order of the king, in the church of St Denis. Du Guesclin lost his first wife in 1371, and married a second in 1373, but he left no legitimate children.
See biography by D.F. Jamison (Charleston, 1863), which was translated into French (1866) by order of Marshal Count Randon, minister of war; also S. Luce, Histoire de B. du Guesclin (Paris, 1876).
DUHAMEL, JEAN BAPTISTE (1624-1706), French physicist, was born in 1624 at Vire in Normandy. He studied at Caen and Paris; wrote at eighteen a tract on the Spherics of Theodosius of Tripolis; then became an Oratorian priest, and fulfilled with great devotion for ten years (1653-1663) the duties of curé at Neuilly-sur-Marne. He was appointed in 1656 almoner to the king, and in 1666 perpetual secretary to the newly founded Academy of Sciences. He died on the 6th of August 1706. He published among other works: Astronomia physica (1660) and De meteoris et fossilibus (1660), both in dialogue form; De consensu veteris et novae philosophiae (1663); De corporum affectionibus (1672); De mente humana (1673); Regiae scientiarum Academiae historia, 1666-1696 (1698), new edition brought down to 1700 (1701); Institutiones biblicae (1698); followed by annotated editions of the Psalms (1701), of the Book of Wisdom, &c. (1703), and of the entire Bible in 1705.
DUHAMEL DU MONCEAU, HENRI LOUIS (1700-1782), French botanist and engineer, son of Alexandre Duhamel, lord of Denainvilliers, was born at Paris in 1700. Having been requested by the Academy of Sciences to investigate a disease which was destroying the saffron plant in Gâtinais, he discovered the cause in a parasitical fungus which attached itself to the roots, and this achievement gained him admission to the Academy in 1728. From then until his death he busied himself chiefly with making experiments in vegetable physiology. Having learned from Sir Hans Sloane that madder possesses the property of giving colour to the bones, he fed animals successively on food mixed and unmixed with madder; and he found that their bones in general exhibited concentric strata of red and white, whilst the softer parts showed in the meantime signs of having been progressively extended. From a number of experiments he was led to believe himself able to explain the growth of bones, and to demonstrate a parallel between the manner of their growth and that of trees. Along with the naturalist Buffon, he made numerous experiments on the growth and strength of wood, and experimented also on the growth of the mistletoe, on layer planting, on smut in corn, &c. He was probably the first, in 1736, to distinguish clearly between the alkalis, potash and soda. From the year 1740 he made meteorological observations, and kept records of the influence of the weather on agricultural production. For many years he was inspector-general of marine, and applied his scientific acquirements to the improvement of naval construction. He died at Paris on the 13th of August 1782.
His works are nearly ninety in number, and include many technical handbooks. The principal are:—Traité des arbres et arbustes qui se cultivent en France en pleine terre; Éléments de l’architecture navale; Traité général des pêches maritimes et fluviatiles; Éléments d’agriculture; La Physique des arbres; Des Semis et plantations des arbres et de leur culture; De l’exploitation des bois; Traité des arbres fruitiers.