METEOROLOGY. 357 



favorable circani stances ^iven continuously since 1870 to the tri-daily 

 weather maps of the United States and Canada should not long ago 

 have hcen supplemented by some general summary of the laws of storm 

 progress and weather changes with wbich the officers of the Signal 

 Service have become familiar. Guided by the writings of Eedfield, 

 Espy, Henry, and Ferrel, the deductive branch of meteorology was 

 making rapid progress — greater than could possibly have been fol- 

 lowed by those outside of the office. It was already in 1872 a matter 

 of common note that storms and also areas of high pressure, &c., pur- 

 sue successively the same or very similar paths, each one showing a 

 progressive systematic difference from its predecessor, until by a sud- 

 den change in distant surrounding circumstances the whole system was 

 broken up and a new order of things inaugurated. These similarities 

 between storm paths were frequently pointed out, both in the daily 

 weather predictions and the Monthly Weather Review. An interest- 

 ing illustration of this will be found in the daily predictions of the 

 hurricanes of August 18 and 25, 1871, as made by the present writer for 

 the official weather "Synopsis and probabilities" of the Army Signal 

 Office.] 



Van Bebber continues by saying that only in the rarest cases are press- 

 ure and temperature distributed around two storm-centers in the same 

 manner, and to this circumstance principally is it to be attributed that 

 the progress of and changes in the depressions show such extraordinary 

 variety. If the distribution of pressure and temperature is reversed, 

 then will the movement of the depression be hindered or entirely an- 

 nulled, and at the same time it itself takes a long irregular form [the 

 barometric trough of the Signal Service BulletinsJ, its longer axis per- 

 pendicular to the pressure gradients, and from its ends frequent small 

 minima break ofl" that then follow the general current of air prevailing 

 in their region ; if, however, on either side pressure or temperature pre- 

 vail, then the direction of the movement of the storm-center will be 

 thereby determined. 



From this short explanation we see the great importance of these two 

 principles in their application to weather predictions. If, however, we 

 would form a correct prediction, we must extend our weather map to the 

 greatest possible extent, especially toward the west, and study expressly 

 the behavior of the great barometric maxima and minima that charac- 

 terize certain regions of the earth. We see also the importance of the 

 study of the cloud movements, especially the ui)per clouds, {Z. 0. G. 

 M., xviii, p. 447.) 



322. Teisserenc de Bort having studied the general weather condi- 

 tions attending abnormal winters, further develops his generalizations 

 relative to the " principal centers of action of the atmosphere," and 

 shows that the perturbations in the position and intensity of these at- 

 mospheric centers correspond with important changes in the chaiacter 

 of the weather. If, therefore, we v ould predii-t th'> weather for a long 



