Most birds acquire their full dimensions in the course of a few months, and are capable of propagation the first summer after they are hatched. In proportion to the size of their bodies, birds possess more vitality, and live longer, than either man or quadrupeds: notwithstanding the difficulties which arise in ascertaining the age of birds, there are instances of great longevity in many of them. Geese and swans have been known to attain to the age of seventy and upwards; ravens are very long-lived birds—they are said sometimes to exceed a century; eagles are supposed to arrive at a great age; pigeons are known to live more than twenty years; and even linnets, and other small birds, have been kept in cages from fifteen to twenty years. * * * * * Every part of their frame is formed for lightness and buoyancy; their bodies are covered with a soft and delicate plumage, so disposed as to protect them from the intense cold of the atmosphere through which they pass; their wings are made of the lightest materials, and yet the force with which they strike the air is so great, as to impel their bodies forward with astonishing rapidity, whilst the tail serves the purpose of a rudder, to direct them to the different objects of their pursuit. The internal structure of birds is no less wisely adapted to the same purposes: all the bones are light and thin, and all the muscles, except those which are appropriated to the purpose of moving the wings, are extremely delicate and light; the lungs are placed close to the back-bone and ribs, the air, entering into them by a communication from the wind-pipe, passes through, and is conveyed into a number of membranous cells which lie upon the sides of the pericardium, and communicate with those of the sternum. In some birds these cells are continued down the wings, and extended even to the pinions, thigh-bones, and other parts of the body, which can be filled and distended with air at the pleasure of the animal.—_Vide_ AIR CELLS. * * * * * It seems evident that this general diffusion of air through the bodies of birds is of infinite use to them, not only in their long and laborious flights, but likewise in preventing their respiration from being stopped or interrupted by the rapidity of their motion through a resisting medium. Were it possible for man to move with the swiftness of a swallow, the actual resistance of the air, as he is not provided with internal reservoirs similar to those of birds, would soon suffocate him. * * * * *