14 



MEMOIRS OP THE NATIONAL ACADEMY OF SCIENCES. 



sensibly equal, as sbown iu Plate VII, which represents the isobars over the eastern i)art of the 

 United States ou the morning of December 9, 187(5. Here we notice a center of high pressure 

 (30.4 inches) ou the western side of the low area, and on that side of the low area the gradients are 

 very steep, and the wind velocities are very high, as shown in the preceding table. 



This table shows the wind velocities reported at 7'' 35"' A. m., but these were uot iu all cases 

 the highest velocities reported during the progress ot this storm. The following table shows the 

 highest velocities reported : 



The winds reported in the neighborhood of New York City are the highest winds that I have 

 found reported at any of the Signal Service stations since the commencement of the observations 

 in 1872, with the exception of Mount Washington. 



Over New England the isobars are very much elongated iu a direction nearly perpendicular 

 to a line joining the high and low centers. Within the isobar 29.2 we find two isobars of 29.1; and 

 at the most northerly' stations the winds ajipear to be mainly controlled by the northern center, and 

 at the more southerly stations the winds are mainly controlled by the southern center. Between 

 two neighboring centers of low pressure we generally find the directions of the wind to be irregu- 

 lar, at certain places being mainly controlled by one of the centers, and at other places being 

 mainly controlled by the other center. 



18. Sometimes within a large area of low pressure we find three centers of cyclonic movement 

 of the winds. These cyclonic centers are generally of unequal depth, but occasionally we find 

 them sensibly equal. Plate VIII shows the isobars over the Atlantic Ocean and Europe on the 

 morning of March 12, 1876. This plate is constructed on the same plan as Plate IV. Here we 

 see three low centers, and the lowest isobar about each of them is 730'"™, and we noti(te a high area 

 (770""") on the west side, another high area (770""") on the southwest side, and a third high area 

 (770""") on the northeast side. The winds over a large portion of this low area are extremely 

 violent, particularly on the south and west sides of the low center which prevails over England, 

 where the winds rise to 6 (on a scale of 1 to 0); and on the northeast side of this center, to a 

 distance of about 360 miles, the winds appear to be controlled by this center. A little further to 

 the northeast the winds are controlled by the low center over Sweden, but on the north side of 

 the Swedish low center the winds are generally feeble, and are mainly controlled by the third low 

 center on the northwest of Norway. 



19. Occasionally, within a large area of low jircssure, we find four or five or even more cyclonic 

 centers, and when the number of centers is so great it seldom occurs that they are all of equal 

 depth. Plate IX shows the isobars over Euro])e and the Atlantic Ocean, on the morning of March 

 9, 1876, The princii)al center of low pressure (71.5"'"') is north of Scotland, and about this center 

 the winds are very violent, rising to number 6 on a scale 1 to 6, and the gradients are steej), 

 particularly on the western side. The cyclonic motion of the winds is strongly marked, the 

 circulation of the winds about the low center being very decided, while the inward tendency is 

 not as great as is generally found in cases where the winds are less violent. On the eastern side 

 of this ])rinci[)al cyclonic center the winds are more feeble, and here we find four minor centers 

 of cyclonic movement. Near St. Petersburg is a low center (740'"'"), about which the cyclonic 

 movement of the winds is distinctly marked. Near the parallel of 50° is a third low center (745™™), 

 where the winds are generally feeble, but they show considerable cyclonic tendency. South of the 

 Black Sea is a fourth low center (75»»"'"'), where the observations are few, but those which are 

 represented ou the chart show a distinct cyclonic tendency; while over the Caspian Sea is a fifth 



