Edinburgh Castle is built on a plug of basalt that solidified in the feeding-pipe of a Lower Carboniferous volcano long since destroyed by erosion. The Castle rock stands high because it is more resistant than the surrounding sedimentary rocks, which dip at a low angle to the east. During the Ice Age, an ice-sheet moved eastwards across the Edinburgh area. Although the ice was so thick that Corstorphine Hill and Arthur’s Seat were only small obstacles in its path, the Castle rock gave some protection to the sedimentary rocks in its lee, which along with the easterly dip, accounts for the mile-long ridge of the High Street and Canongate extending from the Castle down to the Palace of Holyrood. The ridge is accentuated by the horseshoe-shaped hollow gouged by the ice, that is now the site of Princes Street Gardens, the Grassmarket, and the Cowgate. To the east, rising above the intrusive sill of the Salisbury Crags, is Arthur’s Seat, the relic of a tilted and deeply eroded volcano, of which Calton Hill, at the east end of Princes Street, is a fragment.
The ridge from the Castle to the Palace was early chosen as a defensive site. The Nor’ Loch, in the hollow where the railway is today, gave some protection against attack from the north, and a wall protected the town’s other flanks. Thomas Carlyle gave us a picture of the mediaeval city as it was a hundred years before Hutton was born: "Westward on its sheer blue rock towers up the Castle of Edinburgh, and slopes down eastward to the Palace of Holyrood; old Edinburgh Town, a sloping high-street and many steep side lanes, covers like some wrought tissue of stone and mortar, ... with many a gnarled embossment, church steeple, chimney-head, Tolbooth, and other ornament or indispensability, back and ribs of that same eastward slope, – after all not so unlike some crowned couchant animal, of which the Castle were crown, and the life-breath those far-spread smoke-clouds and vapour-clouds rising up there for the last thousand years or so. At the distance of two hundred years or more this thing I see. Rhinoceros Edinburgh lies in the mud: southwards a marshy lake or South Loch, now about to be drained; northwards a marshy lake or North Loch, which will not be drained for the next one hundred and thirty years" (Thomas Carlyle, 1902, p.253).
The population of Edinburgh at the time of the 1991 Census was 418,914. As Arnot observed in 1788 "To ascertain, with any tolerable precision, the number of inhabitants in a great city, is a matter attended with considerable difficulty". Arnot went on to say: "The number of separate families then in Edinburgh, Leith, and their environs, as ascertained by the survey A.D. 1775, amounted to 13,806, which, multiplied. by six, gives the number of inhabitants to be 80,836 (Hugo Arnot, 1788, p.330, 339). Elsewhere we read that the population of Edinburgh, which is thought to have been about 20,000 in 1707, had by 1755 grown to about 31,000 (40,500 including Leith), and to 67,000 in 1800 (Encyclopaedia Britannica, 1929, Vol.7, p.959; Kyd, 1975, p.15; MacKinnon, 1921, p.21; Maitland, 1753, p.220-221; Malcolm, 1951, p.72; Mullay, 1996, p.257-258. For general demography see Flinn, 1977). It is not always clear, unfortunately, to what specific areas the figures given refer. The Old Town, protected by the City Wall, was confined to the upper part of the ridge, on which tall tenements housed a compressed and intimate population. The Old Town is only about one mile long and half a mile wide (Maitland, 1753, with a map dated 1742; Hugo Arnot, 1788, with a map dated 1787). Mossner, Hume’s biographer, likened eighteenth century Edinburgh to modern Manhattan! (Mossner, 1954, p.37).
Visitors from England and the Continent were amazed at the height of the buildings. When Dr Johnson visited Edinburgh on his way to the Hebrides in 1773, Robertson, Ferguson, and Boswell took him on a walking tour. They looked up from the Cowgate to the highest building, "being thirteen floors or storeys from the ground upon the back elevation, the front wall being built upon the edge of the hill and the back wall rising from the bottom of the hill several storeys before it comes to a level with the front wall" (Boswell, 1936, p.25). As James Mackay has remarked, "The skyscrapers themselves symbolised the stratification of Edinburgh society, with the different levels of society occupying the same tenement, but on different floors: the poorer classes occupied the lower storeys, closer to the noise an the smells of the street, while the merchant classes lived in the upper storeys, with the nobility in the flats at the top of the building" (James Mackay, 1992, p.253). The plan of the Old Town resembles the bones of a herring, with the High Street the backbone, supporting closes and wynds (narrow passage ways) at right angles. In the eighteenth century sewage was thrown from the windows, and the High Street was an open drain. Boswell recorded Johnson’s complaint: "I smell you in the dark!" (James Boswell, 1936, p.12).
The Castle dominates the skyline, and the Old Town shelters below; but the last time the Castle’s guns were fired in anger they were aimed at the Town, which had capitulated to Prince Charles’ Highland army.
Great changes took place in Hutton’s adult lifetime. The city no longer needed to be restricted to a defensive position on the ridge. It was time for expansion and fresh thinking. The contrast between the old narrow closes and Robert Adam’s elegant buildings in the New Town illustrate the change. In 1772 the North Bridge was completed, inviting expansion northwards from the city’s medieval bounds. At the north end of the bridge, Robert Adam’s Register House, begun in 1774, made a splendid entrance into the New Town from the Old. David Hume was one of the first to build in the New Town; he moved there in May 1771, before the North Bridge was completed, and it was there that Franklin was his guest.
The South Bridge continued the route of the North Bridge and gave access to the development of new residential areas south of the town. The South Bridge was completed in 1788, just in time for the laying of the foundation stone of Robert Adam’s new building for the University.