142 



THE SUN-SPOT PERIOD. 



emploj^ed, so that while he was occasionally obliged to use series of 

 no more extent than six years of observation, which could not fail to 

 be a serious source of error, I have retained no series shorter than 

 eleven years, corresponding to the mean period of the complete sun 

 spot cycle. Furthermore, while Koppen had no observations from 

 stations outside the Indies, the Antilles, and tropical America, I have 

 been able to employ data from a greater number of stations, distrib- 

 uted more thoroughly over the globe, so that the result obtained can 

 be considered as really representing the mean state of all that portion 

 of the earth comprised within the Tropics. The stations for which I 

 have used all the observations published since 1870 are: 



Sierra Leone, Recife (or Pernambuco), Port au Prince, Trinitd, 

 ffamaica, Habana, Manila, Hongkong, Zi Ka Wei, Batavia, Bombay, 

 Island of Rodriguez; Island of Mauritius. 



For each station there has been computed the deviation of the moan 

 temperature of each year from the general mean for a great number 

 of years. Then for each year from 1870 to 11)00 the general mean of 

 the deviations of all the stations was obtained. 



The following table contains the results thus derived. In the col- 

 unin headed '' Sun spots," will be found for each year the relative 

 number of sun spots according to Wolf; and the column headed 

 "Deviations," gives in degrees centigrade the mean departure in tem- 

 perature for all the stations as obtained in the following manner: 



If aj represent the arithmetical mean of the deviations of tempera- 

 ture at all stations for a given year, a^ that for the ^^ear preceding, 

 and a2 that for the 3'ear following, the number found in the column 

 headed " Deviations " corresponding to the 3'ear in c^uestion is equal to 



a„+2ai+a2 



These numbers have ])een emplo3'ed rather than the direct arith- 

 metical mean for the given year, in order to give a more regular 

 series b}" eliminating as well as possible the secondary irregularities. 



Table I. — Co7nparison of sun spots and temperatures, 1870 to 1900. 



" Maximum. 



