12 E. Loom'ts — ('<>ntrihii.th>ns to Meteorology. 



eight miles an hour at Dodge city ; thirty-six at Yankton, thir- 

 ty-two at Leavenworth, and thirty -one "at Eseanaba. In No. 

 53 the low center had already reached Canada, so that the 

 amount of rain fall on the front side of the low center is 

 unknown. 



Thus in each of these fourteen cases, whenever the observa- 

 tions furnish the requisite information, we find that a heavy fall 

 of rain preceded the low center, and in six of the cases we find 

 a special cause why a small change of pressure carried the 

 low center forward with unusual velocity. Also in six cases 

 we find an area of high pressure on the northeast side together 

 with an area of high pressure on the northwest side, distant 

 from the former about seventeen hundred miles. These facts 

 are regarded as explaining and confirming the remark made on 

 page 10, that the movement of centers of low pressure is mod- 

 ified by the distribution of pressure existing beyond the limits 

 of the low area. 



Among the seventy-four cases in the table on page 2, there 

 are eleven in which the low center advanced at a rate not ex- 

 ceeding sixteen miles per hour, viz : Nos. 2, 7, 8, 18, 20, 33, 

 34, 58, 59, 65 and 73. In Nos. 2 and 8 there was a trough of 

 low pressure extending more than one thousand miles across 

 the United States from southwest to northeast. A considerable 

 fall of rain on the southwest side of the principal center of low 

 pressure, accompanied by a cyclonic movement of the winds, 

 was apparently the cause of a small fall of the barometer on 

 that side, which in No. 2 carried the center of low pressure 

 towards th< south ?est, and in No. 8 towards the southeast. In 

 Nos. 7, 33, 34. 5s, 59, bo and 73 an area of low pressure pre- 

 vailed in the region between the Mississippi Kiver and \ 

 M> • tains, and in some of the cases extended as far as the Pa- 

 cific Ocean. The influence of this low pressure extended east- 

 ward, and diverted the westerly winds which otherwise might 

 have been expected to follow the first low center. This was 

 apparently the reason why these low centers advanced so slowly, 

 and in one case the low was diverted westward. In No. i8 

 the low center was near Fort Sully, but on account of the 

 <mall number of stations ol observation, the exact position of 

 the center cannot be assigned. The observations however, 

 show that for four days, from March 13th to 17th, an area of 

 sure prevailed over the United States east of the &lis- 

 er, while a low pressure prevailed between the merid- 

 ian of 100° and the Rocky Mountains, and this low area main- 

 tained oearlj the same position during these four days. On the 

 afternoon of March 17th, a high appeared on the coast of Ore- 

 gon, which crossed the Rocky Mountains on the 19th, and as 

 it advanced eastward, the low was apparently pushed forward 



