CONTRIBUTIONS TO METEOROLOGY. 15 



low center (755"""). On tlie iiortliwest side of tliis ininiense area of low ])ressnre is au area of higli 

 pressure (775"'"'), a!ul on the southwest side is another area of hit;ii pressure (775'""'). On the east 

 side the highest isobar represented is 705""", but the length of this isobar and its relation to the 

 low i)ressnre on the western side lead us to expect higher i)ressure further east, and by consnlting 

 observations in Asiatic Russia (not represented on tiie niaj)) we find that tiie pressure continued 

 to increase in advancing eastward. 



20. Sometimes we find areas of low pressure of greater extent than any of the preceding, and 

 showing numerous centers about which the winds circulate with considerable force, when the 

 barometric^ dejiression is considerable, but feeble when the dei)ression is small. The international 

 weather maps show numerous examples of this kind. According to these maps, on the morning 

 of June 7, 1882, there was an area of low pressure which covered the whole of Asia, and aitpareutly 

 extended from the equator to a considerable distance beyond the North Pole; it covered the whole 

 of Europe with the exception of a very small i)ortion of its southern margin; it covered the 

 northern part of the Atlantic Ocean, and reached across the central portion of North America to 

 the Pacitic Ocean, extending thus through about 320 degrees of longitude. The principal low 

 center (29.2 inches) was north of the Caspian Sea; a second low center (29.4 inches) was over the 

 northern part of India; a third loiv center (29.6 inches) over the Gulf of Saint Lawrence; a fourth 

 low center (29.8 inches) over China; a fifth low center (29.8 inches) northeast of Japan; and if the 

 observations were sutliciently numerous, there is little doubt that several other subordinate low 

 centers would be exhibited. A center of high pressure (30,4 inches) was found over the Atlantic 

 Ocean near latitude 35°; a second center of high jjressure (30.2 inches) over the southeastern part 

 of the United State.- ; and a third center (30.2 inches) over the easteru part of the Pacilic Ocean 

 near latitude 30°. The area of high pressure formed a belt following the parallel of 30° or 35°, 

 and extending through at least 240° of longitude, but interrupted by the Asiatic Continent. 



21. Plate IX illustrates the tendency to the formation of subordinate centers of cyclonic 

 action, whenever within a very extensive area of low i)ressure the winds are comparatively feeble. 

 When this tendency is slight it simply causes a little distortion in the isobars without the 

 formation of a distinct area of cyclonic action. There are few cases of great storms in which we 

 do not find some distortion of the isobars which may be ascribed to this cause. In Plate III most 

 of the isobars are uncommonly symmetrical, but we notice a distortion of the isobar 30.0 inches 

 over Spain, and the winds in this neighborhood indicate a feeble center of cyclonic action. The 

 same remark is illustrated by Plates IV and VIII. 



22. From an examination of the Signal Service maps, we find that in the United States au 

 area of low barometer, with only one system of cyclonic winds, frequently has a diameter of 1,600 

 English miles. From Hofliueyer's charts we find that over the Atlantic Ocean such an area 

 frequently has a diameter of 2,000 English miles. Areas of low barometer, having several centers 

 of cyclonic action, may have a diameter of 6,000 English miles, and may form a belt extending, 

 nearly (if not entirely) around the globe, between the parallels of 40 and 50 degrees. 



Direction of movement of areas of lotc pressure. 



23. Areas of low pressure seldom remain stationary in position for many hours. The center 

 of low pressure generally changes its position steadily from hour to hour, and everywhere we find 

 a marked uniformity in the direction of this movement. Plate X shows the tracks of a large 

 number of centers of low pressure, for the United States and the adjacent districts. This plate is 

 not designed to indicate the track which storm centers most frequently pursue, but rather to give 

 an example of all the more important tracks j)nrsued by storm centers, as delineated on the Signal 

 Service maps. The tracks represented on the northern part of the chart are those which most 

 frequently occur, while those on the southern part of the chart are comi)aratively infrequent. We 

 perceive that north of the parallel of 30° storms generally travel from west towards the east; 

 but in some places they deviate to the south of east, and in other places they deviate to the north 

 of east. On the southeast poi'tion of the chart we notice several tracks which are directed 

 towards the northwest. 



24. A chart which represents storm tracks for the entire northern hemisphere is best adapted 



