At frequent intervals in the past the subject of the passage from model to full scale has received fairly thorough treatment at the hands of the members of this Society, and although a considerable mass of further material of extreme practical importance has since then accumulated, I do not propose to present it to you this evening. Those who are particularly interested in the data will find the greater part of it discussed in the Reports and Memoranda of the Advisory Committee for Aeronautics. I intend instead to devote this evening to a study of the question from a more theoretical standpoint, and in doing so I make no apology.