OP ARTS AND SCIENCES. 



67 



circulation of cyclonic winds.* The greatest number of our summer 

 thunder-storms occur when a centre of low pressure lies to the north 

 or northeast of us. But there are also examples of storms occurring 

 in areas of high or equable pressure, as on August 1st; these would 

 seem to correspond to the Wdrme-Gewitter of Mohn, and to depend 

 on a local warming of the lower air sufficient to bring about the un- 

 stable equilibrium that in the other cases ( Wirbel - Gewitter) depends on 

 the arrangement of upper and lower currents m the cyclonic circula- 

 tion. A more peculiar exception to the general rule of occurrence is 

 found on certain dates when a well-developed low pressure centre 

 passed north of New England, without bringing any local storms to us. 

 The most pronounced example of this condition was on July 17th, in 

 the middle of a long period remarkably free fi-om storms ; it was one 

 of the hottest days of the summer, and yet passed without reports 

 worth mentioning. Other exceptions have a more visible explanation: 

 on June loth, low pressure centres stood both northeast and northwest 

 of us, and no storms occurred ; on July 10th, low pressure centres were 

 north and south of us, and we had only light rains ; the occurrence of 

 two neighboring low pressure areas may in these cases have interfered 

 with the arrangement of upper winds needed for the development of 

 the heavy clouds of thunder-storms. 



The direction of motion of thunder-storms seems to be about at 

 right angles to the radius joining them with the low pressure centre ; 

 that is, about parallel with the higher winds in which they are borne. 

 This is clearly shown in the following table, embracing thirty-five 

 storms whose course was sufficiently well determined to be used in 

 evidence. 



TABLE VI. 



* Tlie first definite announcement of tlie fact tliat thunder-storms have a 

 definite position in areas of low pressure was made by Marie'-Davy, of the 

 Frencii meteorological service, in 1864 : " The appearance of storms always 

 coincides with the presence of rotary winds known under the name of * bour- 



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