46 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



coiideused, aud is i)recii)ltated iu the foini of rain or fsiiow. The heat, liberated in the condeusa- 

 tiou of this va])Or, causes a stronger upward uioveuient of the air, tlie inward movement of the air 

 is accelerated, aud the barometer falls for a reason to be explained hereafter. Thus may be formed 

 an area of low pressure as exhibited in Plate XIV, Fig. 2, within which the winds circulate with 

 great velocity, and there is an abundant precii)itation of rain or snow. If there is a second area of 

 low pressure within a moderate distance these two systems of circulating winds may unite to form 

 a single system of winds, and thus the two low centers may coalesce. Whenever the first case 

 exists (vnz, two areas of high pressure not very remote from eacli other), there generally results a 

 fall of rain, with the development of a new low center, and this low center under favorable circum- 

 stances will coalesce with another low center in its vicinity. 



62. There remain five cases not included in either of the preceding lists, viz, Nos. 25, 32, 33, 

 38, and 39, and each of these cases bears some reseml)lance to Class IV for the United States, 

 Art. 58. In No. 25 the ]>ressure was below 30 inches throughout the whole of Asia aud a large 

 part of Europe, and there were several centers about which there prevailed feeble systems ot 

 circulating winds. On the western side of the low, near the Ural Mountains, the gradients were 

 feeble aud the pressure on that side was behnv 30 inches for a distance of 2,000 miles. Under 

 such circumstances a small force was sufficient to change the position of the point of least i)ressure. 

 If the isobars for this region could be drawn in a reliable manner for each tenth of an inch they 

 would probably indicate more definitely the nature of this force. 



No. 32 was similar to the preceding. In no [)art of the northern hemisphere did the pressure 

 rise much above 30 inches, aud iu nearly the whole of Euroi>e and Asia the pressure was below 

 30 inches. 



Iu No. 33 the pressure was also below 30 inches in nearly every part of Eui'ope and Asia. 

 These three cases were quite similar, and the abnormal movement of the low center was probably 

 due to the same cause. 



In No. 38 the gradients on the eastern siile were considerable, but on the western side they 

 were slight, and the low center appeared to be crowded westward by an area of high pressure 

 (30.4 to 30.0), which followed it i)ersistently on the northeast side, while the pressure on the 

 southwest side for a great distance was below 30 inches. 



No. 39 was similar to the preceding after the morning of the 21st, and iu each of the last two 

 cases an area of high j)ressure seemed to exert a decided influence in crowding tiie low center 

 westwaid. Each of these five cases was attended by some rainfall, but the amount reported in 

 the published observations does uot seem to i)resent an ade(inate cau.-se tor the abnormal movement 

 of the low centers. 



C3. The preceding discussion seems to warrant the following conclusions, viz : That the 

 westerly movement of low centers, which is occasionally observed in the middle latitudes of Europe 

 and America, is generally due to one or more of the following causes : 



1. The intlnence of one low area upon an adjacent low area, which influence sometimes seems 

 to act as an attractive force. 



2. The influence of a considerable fall of rain or snow-, which also acts as an attractive force. 



3. The influence exerted by two areas of high pressure, uot very remote from each other, by 

 which means a new movement is imparted to the air included between them, and a new low center 

 is sometime developed. 



4. The intiueuce of an area of high (or only moderately high) pressure, on the northeast side 

 of a low area, when the gradients on the southwest side of the low area are slight, iu which case 

 the center of the low area may be crowded towards the southwest. 



If these causes are sometimes sufficiently i)owerful to divert the center of a low area westward, 

 it may be presumed that there are many more cases iu which these causes are sufficiently powerful 

 to aflect, iu an appreciable degree, both the direction and velocity of the movement of a low center. 



04. The facts which have been stated in the preceding ])ages seem to aft'ord a basis for some 

 general conclusions respecting the movement of storm ai'eas. Many meteorologists have claimed 

 that the progressive movement of storm areas is satisfactorily explained by sa.\1ng that they are 

 carried forwajd by the general movement of the mass of the atmosphere within which they are 



