CONTRIBUTIONS TO METEOROLOGY. 



49 



the cases here cited the depression of the barouieter was generally considerable, and tbe position 

 of the low centers was distinctly indicated : 



Cases in which a second tow center is developed within an area of lore pressure. 



In a majority of these cases the two low centers appear to have subsequently coalesced; but 

 in several of them the two low centers moved off in directions inclined to each other at an angle 

 of 90° or more, and with unequal velocities. 



70. Over the Atlantic Ocean and Europe cases similar to tlie preceding are of much more 

 frequent occurrence than in the United States; the depression of the barometer is generally much 

 greater, and the low areas bave a much greater geographical extent. By consulting Hofl'meyer's 

 weather maps we may easily iind examples in which two low centers move towards each other 

 from nearly oi)posite directions and coalesce ; and we may also find frequent cases in which a 

 great area of low pressure, with but one center, undergoes a change by which two low centers are 

 developed, and these new low centers recede from each other. Sometimes there is a further 

 change by which three or four, or even more, low centers are formed, and these low centers have 

 a progressive movement in difleient directions, and with unequal velocities. On the contrary, 

 within a large area of low pressure showing several low centers, a low center may disappear from 

 simple changes of pressure. In like manner a second low center may disappear, and so on. Plate 

 IX shows five low centers within a single area of low i)ressure. The map for the preceding day 

 showed only three low centers within the same low area, while the map for the following day 

 showed five low centers, but one of them had no connection with either of the five shown on Plate 

 IX. In cases like this the changes in the position and magnitude of the low centers are so rapid 

 that, in comparing two weather maps for successive days, we frequently find it impossible to 

 identify a low center on one of the maps, with its corresponding low center on the other maiJ. 



71. Examples may be easily found to illustrate all of these different cases; but for the con- 

 venience of those who may wish to examine such cases without the trouble of searching for them 

 the following lists are given, and the cases are all taken from Hofl'meyer's charts for 1875 : 



1. Examples in which tivo centers of low pressure approach each other and coalesce. 



2. Examples in rchich, within an area of low pressure, two or three low centers are developed where only 



one had existed previously. 



In several of these cases the two low centers appear to have started from the same low center 

 and thence receded from each other. This is particularly true for January 2-3; April 6-7 ; June 

 24-25; September S-9 ; November 11-12 ; November 13-14 ; December 8-9; and December 21-22. 

 S. Mis. 154 7 



