United States Weather Maps. 9 
not only at places where the pressure was above 30 inches, but 
also at places where the barometer had not yet reached the 
height of 80 inches. I conclude, therefore, that the severe cold 
which prevailed at this period throughout the United States 
east of the Rocky Mountains was mainly the result of cold air 
descending from the upper regions of the atmosphere under the 
influence of a high barometer. 
The only other explanation of this phenomenon which I 
think can be plausibly urged, is that this cold was the result of a 
current of air sweeping along the surface of the earth from a 
very high northern latitude, and bringing with it the low tem- 
perature of the region from which it came. I admit that dur- 
ing the period in question northerly winds were unusually 
cause alone will not account for the suddenness and magnitude 
of the depression in the present case. From the 21st to the 22d 
of December, the mean temperature at New Haven fell 15°, 
and it continued at about this point or even lower for nine sue- 
cessive days. Was there at this time a steady flow of air from 
the Arctic regions sufficient to account for this effect? Iam 
unable to appeal to my weather maps for an answer to this 
~ cia for, unfortunately, on these days, the observations 
tom most of the stations in the extreme northwest are wanting, 
and if it were otherwise, it might appear that the maps did not 
extend far enough northward to furnish all the information 
which was required. If our observations covered the whole 
area of North America, I have little doubt we should find that 
the depression of the thermometer below its mean height was 
greater in the United States than it was in the region north of 
us, as I have shown was the case in the storm of Dee. 20, 1836, 
of which the investigation was published in vol. xi of the 
Smithsonian Contributions to Knowledge. 
Storm of Jan. 6-8, 1874. 
In the absence of adequate observations from British North 
America, it may be more satisfactory to take an example of a 
Storm in the southern part of the United States, in which case 
the weather maps will inform us of the condition of the atmos- 
here on the northern margin of the storm. For this purpose, 
have selected the storm of Jan. 6-8, 1874, which came up 
from the Gulf of Mexico and crossed the United States in a 
direction about N.30°E. The following table shows the ob- 
Servations of the thermometer at 7" 35" A. M., from Jan. 4 to 
