The aggregation of so vast a number of persons within so comparatively limited an area--the immense quantity of food required for their daily sustenance, as well as of fuel, clothing, and other necessaries--would be attended with no small inconvenience and danger but for the facilities again provided by the railways. The provisioning of a garrison of even four thousand men is considered a formidable affair; how much more so the provisioning of nearly four millions of people! The whole mystery is explained by the admirable organization of the railway service, and the regularity and dispatch with which it is conducted. We are enabled by the courtesy of the general managers of the London railways to bring together the following brief summary of facts relating to the food supply of London, which will probably be regarded by most readers as of a very remarkable character. Generally speaking, the railways to the south of the Thames contribute comparatively little toward the feeding of London. They are, for the most part, passenger and residential lines, traversing a limited and not very fertile district bounded by the sea-coast, and, excepting in fruit and vegetables, milk and hops, they probably carry more food from London than they bring to it. The principal supplies of grain, flour, potatoes, and fish are brought by railway from the eastern counties of England and Scotland; and of cattle and sheep, beef and mutton, from the grazing counties of the west and northwest of Britain, as far as from the Highlands of Scotland, which, through the instrumentality of railways, have become part of the great grazing-grounds of the metropolis. Take first "the staff of life"--bread and its constituents. Of wheat, not less than 222,080 quarters were brought into London by railway in 1867, besides what was brought by sea; of oats, 151,757 quarters; of barley, 70,282 quarters; of beans and peas, 51,448 quarters. Of the wheat and barley, by far the largest proportion was brought by the Great Eastern Railway, which delivered in London last year 155,000 quarters of wheat and 45,500 quarters of barley, besides 600,429 quarters more in the form of malt. The largest quantity of oats was brought by the Great Northern Railway, principally from the north of England and the east of Scotland--the quantity delivered by that company in 1867 having been 97,500 quarters, besides 24,664 quarters of wheat, 5560 quarters of barley, and 103,917 quarters of malt. Again, of 1,250,566 sacks of flour and meal delivered in London last year, the Great Eastern brought 654,000 sacks, the Great Northern 232,022 sacks, and the Great Western 136,312 sacks; the principal contribution of the London and Northwestern Railway toward the London bread-stores being 100,760 boxes of American flour, besides 24,300 sacks of English. The total quantity of malt delivered at the London railway stations in 1867 was thirteen hundred thousand sacks.