408 RECORD OP^ SCIENCE FOR 1887 AND 1888. 



out the atmosphere. This distribution is very much aft'ectetl by the 

 presence of oceans, continents, mountain ranges, and pkxteaus which 

 determine the irregular distribution of density and the irregalar resist- 

 ances to the winds. Were the coefficient of friction uniform throughout 

 the whole of the earth's surface the distribution of winds and pressure 

 woukl be much simpler, but as affected by friction it is complicated, as 

 is shown by the isobars on the charts of monthly mean values. 



As the elevations throughout the United States must therefore be 

 carefully borne in mind, because of their bearing on the question of their 

 resistances to the motions of the atmosphere, and still more for the 

 thermodynamic reasons shown further on, therefore a hypsometric 

 map is provided. On this map may be introduced relative numbers, 

 changing with the seasons, showing the local frictional resistance at a 

 standard altitude, or the relative drag of the air blowing over different 

 surfaces; approximate estimates of these numbers are given in the 

 Table xvi. 



The increase of wind velocities at various moderate heights above 

 fields of grass, grain, etc., is given by Stevenson and by Archibald, and 

 may be assumed to be as the square root of the altitude. 



Any departure from the normal densities must be followed by a dis- 

 turbance in the flow of air from the denser toward the lighter. 



This disturbed movement of the air produces at once a change in the 

 distribution of barometric pressure, which change becomes greater in 

 proportion to the movement ; the observed barometric changes are thus 

 principally dependent upon the wind, and in daily predictions it is con- 

 venient to use the barometer as an index of what movements are going 

 on in the atmosphere in the absence of observations of temperature 

 and winds above or beyond the limits of our stations. 



The relations between pressure and wind are given in Professor Fer- 

 rel's works, as also in those of Oberbeck and Guldberg and Mohn, from 

 which it will be seen that a very slight difference of i^ressure produced 

 by a very slight difference of density is sufficient to set the atmosphere 

 in motion in the direction of an " initial gradient," the result of which 

 is immediately to produce a vorticose motion and a steeper " barometric 

 gradient " nearly perpendicular to the initial gradient and to the mo- 

 tion of the wind ; these steep gradients accompany all storms, and are 

 exemplified in Ferrel's " Movements on the Surface of the Earth," 1858, 

 "Meteorological Eesearches, part ii," 1877, and "Eecent Advances," 

 1886. 



The resistances to the motion of the atmosphere would, however, soon 

 bring it to rest, and the abnormal isobars would soon disappear or re- 

 lapse into the normal ones, were there not some force at work maintain- 

 ing the disturbance of density and the abnormal motions. A success- 

 ful storm prediction must depend upon the accuracy with which one 

 can determine the amount, location, and effects of the force that main- 

 tains this disturbance. 



