ACADEMY OF SCIENCES. 115 



Here the air was comparatively calm during the 18th, 19th, and until about 

 noon of the 20th, when we fell under the full influence of the Equatorial current, 

 with a strong south wind and heavy rain. On this occasion the Equatorial 

 current readied beyond the southern border of the State, heavy rains falling at 

 San Diego, and more rain falling in the mountains than on the coast. Whilst 

 this western edge of the Polar current was yielding to the Equatorial, we find 

 that its eastern border was moving to the eastward, displacing the Equatorial 

 current that had been prevailing in the Eastern States. As the data I have 

 collected on this point have been obtained only from the few facts telegraphed 

 to the newspapers, they are necessarily imperfect. On the 19 th a cold northern 

 gale was prevailing at Memphis ; on the 20th a very heavy freshet in Maine ; 

 and on the 21st the weather on the Atlantic border was the coldest of the 

 season, evidently caused by the lateral displacement, eastward, of the Polar cur- 

 rent. How far these movements are connected with atmospheric changes taking 

 place on other parts of the surface of the earth, it is difficult at present to say. I 

 would merely remark that at the time of the descent of the Equatorial current 

 on this coast, or about the 8th and 9th of February, very bad weather was 

 experienced on the Atlantic, and on the 12th a northeast wind prevailed over 

 most of the west part of Europe, causing great cold. As regards the existence 

 of these currents in other years, the only data I have been able to compare are 

 some obtained from the Army Medical Reports. I have taken the quantity of 

 rain that fell at Fort Snelling, latitude 44°, longitude 93^, and at West Point, 

 latitude 41^', longitude 74^, in two seasons, corresponding to two of our rather 

 extreme seasons, viz : 49.50 for a dry season, and 52.53 for a wet one. When 

 we are fully under the influence of the Polar current, and but very little rain 

 falls, it would be highly probable that Fort Snelling would be pretty well in the 

 Equatorial current, and would have at least the average amount of rain ; and on 

 the other hand, when we are well under the Equatorial current, and have a wet 

 season. Fort Snelling would probably be well in the Polar current, and would 

 have a dry season. The quantity of rain at the Fort during our rainy months, 

 October to April, inclusive, was in 1849-50, our dry season, 5.28 inches, whilst 

 during the same months 1852-53, our wet season, but 1.95 inches fell, thus 

 supporting the theory of the prevalence of air currents for considerable periods 

 over the same surface. There can be no doubt but that there are two ways in 

 which these different currents of air become mixed, so that moisture is deposited. 

 In the first place, by the upper current breaking through the under current, and 

 thus reaching the earth's surface having its temperature lowered and its vapor 

 condensed. It is this that affords the usual explanation of our rain storms ; 

 but I believe a more careful investigation of the facts will show that the greater 

 part of our storms are due to the mixing of the currents by lateral displacement, 

 at least in the Temperate Zone. These currents, as before stated, often occupy 

 large areas of the earth's surface over which they flow, apparently but slightly 

 disturbed for considerable periods, interfering with each other only at their 

 borders or where they come into contact laterally — here great atmospheric dis- 

 turbances take place ; whilst in the center of the current the weather maintains 

 the character peculiar to the current, whether Polar or Equatorial. In the 



