In “The Reign of Law,” by the Duke of Argyll (first published in 1867), there is a most notable chapter in which the flight of birds is analyzed. In a note the author makes the following statement: “I owe to my father [John, seventh Duke of Argyll] my knowledge of the theory of flight, which is expounded in this chapter. The retired life he led, and the dislike he had of the work of literary composition, confined the knowledge of his views within a comparatively narrow circle. But his love of mechanical science, and his study of the problem during many years of investigation and experiment, made him thoroughly master of the subject.” Every student of the subject of flight should read the interesting work just mentioned. We may not agree with all the conclusions which are reached, yet the author gives most stimulating food for thought. The following paragraphs are among the most striking, showing, as they do, advanced ideas: “In the first place, it is remarkable that the force which seems so adverse--the force of gravitation drawing down all bodies to the earth--is the very force which is the principal one concerned in flight, and without which flight would be impossible. It is curious how completely this has been forgotten in almost all human attempts to navigate the air. Birds are not lighter than the air, but immensely heavier. If they were lighter than the air they might float, but they could not fly. This is the difference between a bird and a balloon.” (p. 130, Am. ed.) * * * * * “No bird is ever for an instant of time lighter than the air in which it flies; but being, on the contrary, always greatly heavier, it keeps possession of a force capable of supplying momentum, and therefore capable of overcoming any lesser force, such as the ordinary resistance of the atmosphere, and even heavy gales of wind. The force of gravitation, therefore, is used in the flight of birds as one of the most essential of the forces which are available for the accomplishment of the end in view.” (p. 131.) * * * * * “The lightness of a bird is a limit to its velocity. The heavier a bird is, the greater is its possible velocity of flight--because the greater is the store of force; or, to use the language of modern physics, the greater is the quantity of ‘potential energy’ which, with proper implements to act upon aerial resistance, it can always convert into upward, or horizontal, or downward motion, according to its own management and desires.” (p. 144.) * * * * *