The population of your city, increasing with such prodigious rapidity; your sultry summers, and the corrupt atmosphere generated in hot and crowded streets, make it a cause of regret that in laying out New York, no preparation was made, while it was yet practicable, for a range of parks and public gardens along the central part of the island or elsewhere, to remain perpetually for the refreshment and recreation of the citizens during the torrid heats of the warm season. There are yet unoccupied lands on the island which might, I suppose, be procured for the purpose, and which, on account of their rocky and uneven surface, might be laid out into surpassingly beautiful pleasure-grounds; but while we are discussing the subject the advancing population of the city is sweeping over them and covering them from our reach. If we go out of the parks into the streets we find the causes of a corrupt atmosphere much more carefully removed than with us. The streets of London are always clean. Every day, early in the morning, they are swept; and some of them, I believe, at other hours also, by a machine drawn by one of the powerful dray-horses of this country. Whenever an unusually large and fine horse of this breed is produced in the country, he is sent to the London market, and remarkable animals they are, of a height and stature almost elephantine, large-limbed, slow-paced, shaggy-footed, sweeping the ground with their fetlocks, each huge foot armed with a shoe weighing from five to six pounds. One of these strong creatures is harnessed to a street-cleaning machine, which consists of brushes turning over a cylinder and sweeping the dust of the streets into a kind of box. Whether it be wet or dry dust, or mud, the work is thoroughly performed; it is all drawn into the receptacle provided for it, and the huge horse stalks backward and forward along the street until it is almost as clean as a drawing-room.