E. Loomis — Contributions to Meteorology. 251 



sions of the barometer sometimes occur without rain, or at 

 most with very little rain. This result is confirmed by 41 

 months of Signal Service tri-claily observations, which furnish 

 130 cases in which the total rain-fall, at all the stations east of 

 the Rocky Mountains, was less than one-tenth of an inch in 8 

 hours. For a period of 40 hours from October 19.3 to 21.1, 

 1872, the total rain-fall, at all the stations east of the Rocky 

 Mountains, was only O'll inch. An area of low pressure pre- 

 vailed throughout the northwest, and the barometer at St. Paul 

 fell to 29'57. The temperature at LaCrosse rose 27° above the 

 normal. Throughout this low area not a drop of rain was 

 reported during these 40 hours. For a period of 32 hours from 

 May 6.3 to May 7.3, 1874, the total rain-fall at all the stations 

 east of the Rocky Mountains was only 0*07 inch. The barom- 

 eter at Fort Sully fell to 29*44, and the temperature rose 22° 

 above the normal. From May 6.1 to May 8.3, a period of 72 

 hours, the rain-fall within the area of low pressure was only 

 0*09 inch. It may be said that these cases of low pressure 

 generally occurred in that region where the stations of observa- 

 tion are widely separated, and that rain may have fallen at 

 intermediate points where there was no observer. The long 

 continuance of the rainless condition, in the cases just men- 

 tioned, is pretty conclusive evidence that the fall of rain must 

 have been very slight over the entire area of low pressure. 

 The month of October, 1872, was one of unusual drought 

 throughout the whole of the northwestern part of the United 

 States. During the first 27 days of the month, no rain fell at 

 St. Paul, there was only - 02 inch at Fort Sully, and only 0*09 

 inch at Omaha, From Feb. 8.2 to 10.3, 1877, a period of 64 

 hours, the total rain-fall at all the stations east of the Rocky 

 Mountains was only 0*62 inch. An area of low pressure pre- 

 vailed throughout the northwestern part of the United States 

 (Bar. 29 '51 at Bismark), and within this low area not a drop 

 of rain was reported during these 64 hours. The thermometer 

 at St. Paul rose 23° above the normal. 



12. These examples are sufficient to show that in the north- 

 western part of the United States (east of the Rocky Moun- 

 tains) there are sometimes formed areas of low pressure having 

 great geographical extent, and accompanied by an amount of 

 rain which is extremely small. Throughout nearly the whole 

 extent of these low areas the average temperature was above 

 the normal, and in the neighborhood of the low center it was 

 20° above the normal. We cannot ascribe this unusual tem- 

 perature to the heat developed in the condensation of aqueous 

 vapor. We must ascribe it to the direct effect of the sun's 

 rays acting upon the sandy soil of the northwestern plains. 

 These low areas in the northwest were all attended by an area 



