134 ROCHESTER ACADEMY OF SCIENCE. [May 9^ 



The President now introduced Dr. i\I. A. Veeder, who read a 

 paper entitled : 



THUNDERSTORMS, 



By M. a. Veeder. 



The present study of thunderstorms has grown out of the 

 research in regard to auroras and their associated conditions, some of 

 the results of which have been presented before the Academy and 

 published in the proceedings in previous years. In general it has 

 been found that auroras and their attendant magnetic storms occur 

 when spots, or facul?e, or both are at the sun's eastern limb, and near 

 the plane of the earth's orbit. Inasmuch as the proofs of this prop- 

 osition have an important bearing upon the subject of thunderstorms, 

 and may not be accessible to many receiving this paper, it is necessary 

 to rehearse them somewhat at length. 



By counting the stations reporting auroras each day and arrang- 

 ing the numbers thus obtained in periods, it is found that the time of 

 recurrence is twenty-seven days six hours and forty minutes. This 

 result was obtained within four minutes from magnetic observations 

 alone. The four minutes were added after it had been discovered 

 that this slight lengthening of the period would secure conformity 

 with Carrington's determination of the time of a synodic revolution 

 from the average rate of rotation of spots, which value also has been 

 adopted at the Greenwich Observatory. 



So far as is known to the writer this determination has never been 

 made in the case of magnetic phenomena with such accuracy here- 

 tofore. Round numbers and general statements based upon records 

 from too few stations or for too short a time, are entirely inadequate 

 for the purposes of the present investigation. It is not enough to 

 know that there is recurrence at intervals of about a month. The 

 limits of probable error should be less than any considerable fraction 

 of even a single day. The tables upon which the period that has 

 been named is based cover nearly two hundred years, and comprise 

 nearly all reports of auroras in existence, as well as records of mag- 

 netic storms for a considerable number of years. These tables are 

 so voluminous that only specimen extracts, such as that appended 

 to the paper upon the Zodiacal Light in the Proceedings last 

 year, have been published. This extensive and thorough tabula- 

 tion has reduced the limit of probable error to a question of a few 

 minutes more or less. An error amounting to as much as half a day 

 would multiply itself so as to become a whole week in the course of 



