252 E. Loomis — Contributions to Meteorology. 



of liigli pressure on the east or southeast side, where the aver- 

 age height of the barometer was 30*36 inches. This high tem- 

 perature over the northwestern plains, combined with an area 

 of high pressure on the east or southeast side, is sufficient to 

 cause a general movement of the surrounding air towards the 

 heated region. The Signal Service maps are too limited in 

 extent to show what were the atmospheric conditions on the 

 north and west sides of the low area ; but we must conclude 

 that the colder air from the north would press down to dis- 

 place the warmer air of the low area. Thus we find all the 

 forces requisite to generate a cyclonic movement of the winds 

 about the center of the heated region. The heat of the central 

 area is continually recruited by the direct action of the sun's 

 rays, and thus the cyclonic movement of the low area may be 

 maintained for a long time, while the steady pressure of the 

 air on the western side (arising from the same causes which 

 determine the average system of circulation of the winds) fills 

 up the low area on its western side, and thus crowds the low 

 area slowly eastward. 



13. If areas of low pressure of great geographical extent 

 may be formed and maintained for several days with very little 

 rain, is there no difference between the low areas which are 

 attended by a heavy rain-fall, and those which are attended by 

 very little rain ? Differences do exist and generally they are 

 strongly marked. The following are some of them : 



Characteristics of areas of low pressure 



With, excessive rain-fall. 



a. Steep barometric gradients. 



b. Violent winds. 



c. Rapid changes of barometric 



pressure. 



d. Rapid progressive movement. 



With little or no rain. 

 a. Feeble barometric gradients. 

 5. Moderate winds. 



c. Slow changes of barometric 

 pressure. 



d. Slow progressive movement. 



These are the characteristics of an area of low pressure which 

 stands alone, uninfluenced by the proximity of a second area of 

 low pressure. When two areas of low pressure are formed 

 near each other, their movements are often very complicated. 



14. In my eighteenth paper, I investigated the relation of 

 rain-fall to barometric pressure in Europe, as shown by the 

 observations contained in the International Bulletin. A more 

 extensive comparison of observations has led to conclusions dif- 

 fering but little from those stated in my former paper. 



If we could have similar observations of the rain -fall over all 

 parts of the Atlantic Ocean, they would be of great value in 

 determining the influence of rain-fall upon barometric pressure. 

 As such observations have never been made, I have sought for 

 the best available information bearing upon this question. 



