changes of volume and pressure and corresponding changes of temperature, moisture and cloud. These studies are comprised under the technical terms hydro-mechanics, aero-dynamics and thermo-dynamics. The atmospheric problems of to-day and of all future time will undoubtedly be concerned principally with these three classes of questions, and another century may elapse before men can solve them all.
I have here at hand a circular table which represents a small portion of our globe within the polar circle, while the center of the table represents the north pole itself. I will set the table in rotation; of course it revolves much more rapidly than the earth does. It is now revolving from left to right as the earth itself does when we stand facing the north and see the sun rising in the east. If I shoot this ball so that it rolls straight across the revolving table, it will not trace a straight line on that table but a curved line. If the track lie on the right-hand side of the pole the curvature will be toward the equator, but if on the left-hand side then the curve will be away from the equator. In both cases the curvature is toward the right hand as the ball progresses. This sentence corresponds to the two cases of a body moving, respectively, eastward or westward on the earth. When it moves eastward it has a greater centrifugal force than the corresponding point on the globe and pushes toward the equator. When it moves westward it has a less centrifugal force and retreats toward the pole. Corresponding phenomena occur when a pendulum is swung to and fro as in the Foucault pendulum experiment; or when a gyroscope is rapidly spun, as was also done by Foucault. We were long since taught by Poisson, Tracy and Ferrel that any mass, whether solid, liquid or gaseous, moving on the surface of the rotating earth in the northern hemisphere experiences a deflection to the right, and this is true under ordinary circumstances. Perhaps you will not be surprised to learn that our distinguished mathematical colleague. Professor Chessin, of Washington University, St. Louis, has lately reopened this question and even yesterday in this very lecture room showed that under some circumstances the deflection may be to the left, so that questions which have been considered settled for many years are now deemed worthy of a new investigation. Thus in meteorology we must expect to be frequently called upon to revise our old ideas in the light of the newest researches.
The study of hurricanes and typhoons long ago led to the general conclusion that they consist of comparatively thin layers of air revolving horizontally in nearly circular orbits, and therefore analogous to the revolving horizontal wheel of a gyroscope whose axis is vertical. On the other hand the phenomena of the waterspout at sea and the tornado on land had led to the idea that in these cases we have to do with nearly vertical ascending currents of air. Redfield's careful construction of numerous weather maps made him certain that in a hurricane the winds
