710 



SCIENCE. 



[N. S. Vol. ir. No. 



Columbia College can do to provide for the 

 meteorological needs of this country. The 

 mere statement of these subjects — the three 

 lines of tj-pe that show the student what he 

 may study if he will — serves as a suificient 

 stimulus, if his bent is in that direction. 



I maintain that there is a real demand for 

 a broad course of instruction in meteorology 

 and that there is an abundance of work to be 

 done, both mathematical and experimental. 

 The courses and the laboratory work that 

 bear on the study of the atmosphere are al- 

 most the same as those that one would nat- 

 urally take up if one were preparing to be a 

 hydraulic engineer. The fundamental ques- 

 tion to be resolved in the study of the me- 

 chanics of the atmosphere consists in de- 

 termining what the general motions of the 

 air must be under the influence of gravitation 

 and the rotation of the earth; of evaporation 

 and condensation of moisture ; of absorption 

 and radiation of heat, and of the irregu- 

 larities of oceans and continents, hills and 

 valleys. If there were no solar heat the 

 temperature would be fairly uniform at all 

 altitudes, the earth and the sea would be 

 frozen, there would be no clouds, and the 

 atmosphere would be a stagnant layer re- 

 volving with the globe to which it adheres. 



Professor William Ferrel, a native of 

 Pennsylvania, was the first to solve ap- 

 proximately the equations of motion and 

 deduce some of the phenomena which as ob- 

 servation shows actually exist. He proved, 

 that any free body in motion on a rotating 

 surface would be deflected to the right in 

 the northern hemisphere, and that a pres- 

 sure in that direction would therefore ac- 

 company any effort to make the body move 

 in a straight line. In consequence of this 

 deflection a belt of low pressure must exist 

 around the earth at the equator and areas 

 of low pressure at the poles with special 

 areas of high pressure at the tropics. 

 Among the equations of fluid motion Ferrel 

 included the ' equation of continuity ' so- 



called, but found that the general solution 

 of the problem as thus stated analytically 

 was impracticable; he therefore took as a 

 special solution the observed pressures and 

 temperatures all over the globe and showed 

 what the relative motions must be both 

 for the lower winds and the upper atmos- 

 pheric currents. He then proceeded to a 

 discussion of the temperatures, pressures 

 and winds that must be experienced within 

 a region of abnormally high or low pres- 

 sure, such as we now call cyclones or anti- 

 cyclones. He derived the formula connect- 

 ing the intensity of the barometric gradients 

 with the winds that cut across them diag- 

 onally. 



Ferrel's next memoir took up the thermo- 

 dynamic problems, especially those that 

 Espy had seen to be important factors in 

 the development of our thunderstorms, 

 showing that ascending air expands and by 

 virtue of its expansion is cooled through- 

 out its whole mass to an extent easily cal- 

 culated by the laws of thermo-dynamics, 

 and that when cooled below the dew point 

 a formation of fog and cloud must result, 

 giving rise to an evolution of heat and a 

 delay in the cooling process, so that the 

 moist ah- is warmer than the dry air would 

 have been. Thus a cloud once formed be- 

 comes a center of aspiration, so that clouds 

 and storms grow as long as they are sup- 

 plied with uprising currents of moist air. 

 Ferrel reduced to formulfe and figures the 

 general doctrines of Espj^ and showed them 

 to be perfectly applicable to a certain class 

 of our storms, namely, those in which the 

 ascent of air is sufficiently rapid to render 

 the radiation of heat and the mixture with 

 surrounding air matters of secondary im- 

 portance. 



The general treatise of Professor Ferrel 

 entitled ' Recent Advances in Meteorology,' 

 published by the Signal Office in 1885, gives 

 most of his earlier results with many re- 

 visions and new ideas. 



