METEOROLOGICAL SUB-CONDITIONS. 5 



tions in the distribution of heat, either of territorial action, or from equinoctial 

 shiftings or other suppositions, none can be applied with a dominant purpose 

 of practical utility. 



Of the immediate effect, however, of the passage of one of these areas of 

 lower pressures the conditions follow very regularly. As the depression ap- 

 proaches from the west or southwest, the wind begins to blow from the southeast 

 to east, increasing in force as the greatest gradient is reached. If the area passes 

 south of a station, the wind follows it round from the northeast to north and 

 northwest. If it passes to the north of the station, the wind takes a reverse 

 direction from the southeast to south and southwest, the velocity of the following 

 wind being dependent upon similar differences of pressure as that which acted 

 during its approach. 



The rainfall, it is to be noticed, depends rather upon local conditions, as 

 the greater or less humidity which the winds possess from passing over areas of 

 snow or bodies of water. It frequently happens that a wind partially saturated 

 receives sufficient additional moisture to produce precipitation over a damp dis- 

 trict, which would be inoperative over a dry one. 



The normal action of an area of low barometers seems to depend upon its free 

 passage in the line of its natural direction. If this course is interrupted, as by 

 the opposition of a body of air of greater pressure, so that its movement is 

 delayed or wholly arrested, the tendency is to narrow its dimensions, sometimes 

 to turn it aside, and generally to increase its destructive cyclonic tendencies, 

 while not infrequently a second area is developed at some distance to the south, 

 which moves obliquely to join it. The contrary effect of a dissipation of a 

 marked area results when opposition to its movements becomes removed. Its 

 force is then expended in a lateral expansion. 



Perhaps this dissipation is the result of a secondary formation, which causes 

 a transference of direction of pressure, for if we look at the line of a track of 

 low barometer we cannot fail to be struck by its valley-like line. It is as if it 

 were the track of a marble among shifting elevations of an agitated soil. 



Whatever electrical influences may accompany the passage of a storm area 

 their accessory nature seems quite well proven. Their intensity is certainly 

 associated with peculiar conditions of the atmosphere rather than with variations 

 in its pressure, as indicated by the barometer, and if it can be shown that the 

 mechanical action of the winds is the chief element of these disturbances there 

 will be renewed the extraneous and conflicting theories which now distant the 

 gathered facts and prevent accurate formulating of simple mechanical causes for 

 their results. 



