But anyone who takes an intelligent interest in the structure of the atmosphere, whether he regards it in detail or thinks only of its more general features, must have a working knowledge of the physical properties of air, and that is no slight matter. Maxwell's wonder ful Theory of Heat with all its digressive chapters might have been written for the purpose, for all that it contains is extraordinarily appropriate. Only one additional chapter, on the difficult subject of the thermodynamical properties of moist air, is required. The physical properties of air form the subject of Part II (vol. III). Part III (vol. IV) contains the formal setting out of the dynamical and thermal principles upon which theoretical meteorology depends, and which find their application in Part IV. It is necessarily technical but again its main outlines are sketched by the hand of a perfect master of the art in Maxwell's Matter and Motion. The whole is preceded by a historical introduction and a state ment of the position of the general meteorological problem at the present day (vol. I); because the history of the study of weather forms a striking example of the interaction of the progress of science and the creation of the instruments which it uses. Part IV is issued in advance because what is contained therein has not hitherto been presented in a collected form. It represents the progress made chie?y by those who have been associated in the work of the Meteorological Office in the past twenty years. We owe our success to the fortunate circumstances of our meteorological terrain. For the other parts of the subject all meteorologists have the same sources of reference open to them if they care to use them.