32 TEE POPULAR SCIENCE MONTHLY 



changes of volume and pressure and corresponding changes of tempera- 

 ture, moisture and cloud. These studies are comprised under the tech- 

 nical terms hydro-mechanics, aero-dynamics and thermo-dynamics. 

 The atmospheric problems of to-day and of all future time will un- 

 doubtedly 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 repre- 

 sents tlie 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 cen- 

 trifugal force and retreats toward the pole. Corresponding phenomena 

 occur when a pendulum is swung to and fro as in the Foucault pendu- 

 lum 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 meteor- 

 ology 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 



