246 E. Loomis — Contributions to Meteorology. 



rain-fall exceeded two inches, and at 34 stations the rain-fall 

 was less than one inch. In all of the cases in which the rain- 

 fall did not amount to one inch in 24 hours at any of the 

 stations, the center of low pressure was over the Atlantic 

 Ocean, or very near the coast. In these cases the eastern seg- 

 ment of the low area was over the Atlantic Ocean where the 

 amount of the precipitation could not be measured ; and we 

 have found that the greatest rain-fall almost invariably occurs 

 in this eastern segment. When the low center was over the 

 interior of the continent, the average rain-fall at the principal 

 rain centers was 2 '48 inches ; so that it seems reasonable to 

 conclude that if we had observations from all parts of each of 

 the 131 low areas, the average rain-fall for the principal rain 

 centers would not be less than two and a half inches in 24 

 hours. Even this amount does not seem very large in com- 

 parison with the rain-fall accompanying an average depression 

 of 29 - 63 inches; and we seem forced to conclude that a mod- 

 erate depression of the barometer is as favorable to great rain- 

 fall as an extremely great depression. This may appear to 

 indicate that rain-fall has but little connection with barometric 

 depressions. It should, however, be remembered that the 

 depression at the center of a low area depends not merely upon 

 the barometric gradient, but upon the geographical extent of 

 the low area. If at the center of a low area having a diameter 

 of 1000 miles, the depression of the barometer is one-half inch 

 below the mean, at the center of a low area having a diameter 

 of 2000 miles, with the same barometric gradient, the depres- 

 sion would be an entire inch below the mean. In the United 

 States, when the barometer sinks below 29 inches, the average 

 diameter of the areas of low pressure is 2140 miles ; but when 

 the lowest isobar is 29-5 inches, the average diameter of the 

 low areas is 1185 miles; which shows that when the barometer 

 is most depressed, the average barometric gradient is but little 

 greater than it is with a moderate depression. Extreme 

 depressions of the barometer are generally due to an unusual 

 geographical extent of the low areas, and it appears that great 

 rain-falls depend upon the barometric gradient more than they 

 do upon the geographical extent of the low areas. 



5. I next examined those cases in which the total rain-fall 

 for all the stations was uncommonly great. From September, 

 1872, to November, 1873, I selected those cases in which the 

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

 amounted to at least nine inches in eight hours ; from Decem- 

 ber, 1873, to January, 1875, 1 selected those cases in which the 

 total rain- fall amounted to at least ten inches in eight hours ; 

 from January, 1877, to June,, 1877, eleven inches; and from 

 July, 1877, to December, 1877, twelve inches in eight hours. 



