408 RECORD OF SCIENCE FOR 1887 AND 1888. 



out the atmosphere. This distribution is very much afi'ected by the 

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

 determine the irregular distribution of density and the irregular resist- 

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

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

 would be much simpler, but as afferted 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 

 thermo dynamic reasons shown further on, therefore a hypsometric 

 map is provided. On tbis 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 ba,rometric 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 tbe absence of observations of temi^erature 

 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 pressure 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 " Keceut 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- 

 taiuB this disturbance. 



