When a disease--such as the small-pox or scarlatina--breaks out in one of those crowded houses, and in a densely populated neighbourhood, the consequences are frightful: the mortality is as rapid as that which follows the footsteps of the plague! These are the fearful mysteries of that hideous district which exists in the very heart of this great metropolis. From St. John-street to Saffron Hill--from West-street to Clerkenwell Green, is a maze of narrow lanes, choked up with dirt, pestiferous with nauseous odours, and swarming with a population that is born, lives, and dies, amidst squalor, penury, wretchedness, and crime. Leading out of Holborn, between Field Lane and Ely Place, is Upper Union Court--a narrow lane forming a thoroughfare for only foot passengers. The houses in this court are dingy and gloomy: the sunbeams never linger long there; and should an Italian-boy pass through the place, he does not stop to waste his music upon the inhabitants. The dwellings are chiefly let out in lodgings; and through the open windows upon the ground-floor may occasionally be seen the half-starved families of mechanics crowding round the scantily-supplied table. A few of the lower casements are filled with children's books, pictures of actors and highwaymen glaringly coloured, and lucifer-matches, twine, sweet-stuff, cotton, &c. At one door there stands an oyster-stall, when the comestible itself is in season: over another hangs a small board with a mangle painted upon it. Most of the windows on the ground-floors announce rooms to let, or lodgings for single men; and perhaps a notice may be seen better written than the rest, that artificial-flower makers are required at that address. It was about nine o'clock in the evening when two little children--a boy of seven and a girl of five--walked slowly up this court, hand in hand, and crying bitterly. They were both clothed in rags, and had neither shoes nor stockings upon their feet. Every now and then they stopped, and the boy turned towards his little sister, and endeavoured to console her with kind words and kisses. "Don't cry so, dear," he said: "I'll tell mother that it was all my fault that we couldn't bring home any more money; and so she'll beat me worst. Don't cry--there's a good girl--pray don't!" And the poor little fellow endeavoured to calm his own grief in order to appease the fears of his sister. Those children had now reached the door of the house in which their mother occupied an attic; but they paused upon the step, evincing a mortal repugnance to proceed any farther. At length the little boy contrived by promises and caresses to hush the violence of his sister's grief; and they entered the house, the door of which stood open for the accommodation of the lodgers. Hand in hand these poor children ascended the dark and steep staircase, the boy whispering consolation in the girl's ears. At length they reached the door of the attic: and there they stood for a few moments.