at once loose, rough, and perishable, expensive, tedious, and dangerous to travel on, and very costly to repair.’ At his own expense, and in face of much prejudice, McAdam began at Sauhrie a long course of experiments, and he continued them at Falmouth. He thus arrived at the conclusion that roads should be constructed of broken stone. The surface of the ground on the track of the intended roads was to be raised slightly above the adjoining land; suitable drains were to be formed on each side of the track; it was to be covered by a series of thin layers of hard stone broken into angular fragments of a nearly cubical shape, and as nearly as possible of the same size; no piece was to weigh more than six ounces. The layers of broken stone were to be consolidated gradually by passage of traffic over the road, and the covering of the road would thus become a firm and solid platform, nearly impervious to water, and durable in proportion to the hardness of the stone of which it was made (cf. Imp. Dict. of Biog.) Granite, greenstone, and basalt was at first thought best suited for the purpose; but basalt proved ineffective.
In 1815 McAdam became surveyor-general of the Bristol roads, and he at once put his theories into practice on the highways of his district. He soon directed wider attention to his process by publishing in 1819 ‘A Practical Essay on the Scientific Repair and Preservation of Roads.’ In 1820 there followed his ‘Present State of Road-making,’ and in 1822 this work reached a fifth edition, which was issued ‘with additions and appendix.’
By 1823 the success of the macadamisation of highways was generally recognised, and the question arose whether the system could supersede the rubble-granite causeways in large towns. A committee of the House of Commons was appointed to consider the matter on McAdam's petition, and he gave important evidence in favour of the extended application of his process. The committee adopted his views. In the course of his evidence he stated that between 1798 and 1814 he had travelled over thirty thousand miles of roads in Great Britain in order to pursue his investigations, had spent two thousand days on these tours of inspection, and had expended more than 5,000l. In 1827 McAdam was appointed general surveyor of roads. Parliament voted him an indemnity for past outlay, and a gratuity of 2,000l.—10,000l. in all, but he declined an offer of knighthood.
Though residing thenceforward in Hoddesdon, near Hertford, McAdam continued to pay yearly visits in the summer and autumn to Scotland, and repeatedly revisited the scenes of his boyhood. He usually travelled in a closed carriage drawn by two horses, followed by a Newfoundland dog and a pony, which was wont to carry him to any spot off the main roads that excited his passing interest. While returning from one of these expeditions he died at Moffat, Dumfriesshire, on 26 Nov. 1836, in his eighty-first year.
McAdam married twice. His first wife, whom he married in New York, was daughter of an American settler named Nichol. The maiden name of his second wife, who was also of American descent, was De Lancy. By his first wife he had four sons and three daughters, and by his second he had no issue. His third son, James Nicoll McAdam (1786–1852), accepted in 1834 the knighthood which his father had declined, and was the chief trustee and surveyor of the metropolitan turnpike roads. McAdam's eldest son, William, predeceased him by a few months, leaving a son William (1803–1861), an engineer of ability, and for many years surveyor-general of roads (Gent. Mag. 1861, pt. ii. p. 455). The latter's grandson, William Edward McAdam of Ballochmorie, parish of Colmonell, Ayrshire, is the present head of the family.
McAdam was personally of high and generous character, possessing, it is true, the Celtic warmth of disposition, and outspoken in speech when censure was deserved, yet courteous and amiable in the ordinary relations of life, and a fast friend. He was genuinely interested in science, and was a good writer.
McAdam's efforts largely contributed to produce that network of mail-coach communication which, for some years before railways were introduced, greatly advanced the nation's prosperity and prepared the way for the railway system. McAdam's process was adopted in all parts of the civilised world. The name of the inventor became the synonym for the invention, and derivatives like ‘macadamise’ were universally accepted. In 1824 Southey doubtfully foretold that ‘macadamising the streets of London is likely to prove quackadamising’ (Correspondence, v. 103), but in the same year Miss Mitford warmly eulogised ‘a specimen of macadamisation’ (Our Village, 2nd ser. p. 242), and declared that ‘the Mac-Adam ways are warranted not to wear out’ (ib. 1st ser. p. 231). Jeremy Bentham, in his ‘Rationale of Reward,’ p. 88, claimed in 1825 that ‘MacAdam's system justified the perpetuation of MacAdam's name in popular speech.’ In 1839 Murchison called the makers of the roads ‘Macadamites’ (Silurian System, pt. i. p. 535), and Bailey, in his ‘Festus,’ sc. v. p. 82, expressed anxiety ‘to macadamize the world.’ Moore and Hood likewise helped to