12 



MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



isobars was directed towards each of tbe ten-degree intervals of azimuth, counting from the north 

 point around by east towards the south : 



We see that over the Atlantic Ocean the directions of the longest diameters of the isobars are 

 somewhat more equally distributed in azimuth than they are in the United States; nevertheless 

 the quadiant from SOo to ITOOof azimntb contains only one third of the wliole number of cases, and 

 the center of the region of greatest frequency is N. 35° E., which corresponds almost exactly with 

 the direction already found for the United States. 



12. If we examine the cases in which the elongation of the isobars is greatest we shall tiiid 

 that the position of the longest diameter of the isobars is intimately connected with the position of 

 the neighboring areas of high pressure. Plate IV shows the isobars for May 31, 187.5, over the 

 Atlantic Ocean and Northern ICurope. This plate is coi)ied from the series of Uanisli weather 

 maps which were issued by Cai>tain Hofl'meyer from December, 1873, to No\ ember, 187li, and which 

 are now continued under the joint sui)ervision of the Danish Meteorological Institute and the 

 Hamburg Meteorological Observatory. The isobars represent the atmospheric pressure in 

 millimeters, and are drawn at intervals of 5""". The barometric observations are all reduced to 

 sea-level, and to the temiierature of zero on the centigrade thermometer. Isobars less than 760'"™ 

 are represented by broken lines; isobars of 760""" and n])wards are represented by continuous 

 lines. The direction of the wind is indicated by arrows, and its force is indicated by the number 

 of feathers on the tail of the arrow, according to a scale of 1 to 6 (1 representing the feeblest wind 

 and 6 the strongest). 



We find on this i)late an area of low ]>ressure stretching from SW. to NE. over a distance of 

 4,000 miles, and having a breadth from NW. to SE. of 900 miles. The lowest isobar is 740".'"' 

 (20.13 inches). On the north side of this low area is an area of high pressure (highest isobar 

 shown on the map being 770'"'" or 30.32 inches). On the south side is also an area of high i>res- 

 sure (highest isobar 765'""' or 30.12 inches), and on the east side is a third an-a of high pressure 

 (highest isobar shown on the map being 770""" or 30.32 inches). The situation of these high areas 

 with reference to the low area is somewhat similar to that represented on Plate II. 



13. Within the trojiics we occasionally find areas of low pressure in which the winds are very 

 violent and the gradients are very steep, but the geogra]jhical extent of the low area is much less 

 than in the great storms of the middle latitudes. Plate V shows the isobars during a violent storm 

 which passed over the Philii)pine Islands (Asiatic Archijielago) November 5, 1882. On the north 

 and south sides the gradient amounted to 9""" for a half degree, which is at the rate of 18'"'" (=:0.71 

 inch) for one degree ; and this is equal to the steepest gradient shown on Plate III. The greatest 

 velocity of the wind re|H)rted was 45 meters pe'r second, or 100 miles per hour, which is greater 

 than any wind reported during the storm repiesented on Plate HI ; yet the diameter of this low 

 area, measured from north to south, did not much exceed 500 miles. The cyclones of the tro])ics 

 frequently exhibit a violence greater than is ever known in the storms of the middle latitudes, but 

 their geographical extent is comparatively small. It will be noticed that on Plate V the winds all 

 incline inward, as on Plates I and III, and show a tendency to circulate about the low center from 

 right to left, but the inclination inward is more strongly marked than in most storms of the middle 

 latitudes. 



14. The lower i^ortion of Plate V exhibits the changes of barometric pressure, and also the 

 changes in the direction and force of the wind as shown by self-registering instruments at Manila 

 iluring the progress of the storm, this place being situated very near the path of the center of low 

 pressure. The pressui'e in millimeters is indicated on the left margin of the plate; the velocity of 

 the wind is shown on the left margin in meters per second ; the hours are shown at the top of the 



