SUNSPOTS AND WEATHER. 509 



important to know how much dependence can be placed on the evidence 

 for the existence of a similar period in the sunspot observations. In 

 1900 Dr. Lockyer took Wolfs sunspot numbers from 1833 to 1900; 

 these numbers show six maxima and six minima of the average eleven- 

 year period. But the interval from a minimum to the next following 

 maximum is not the same for each of these six periods ; it varies from 3^ 

 years to about 4^ years. The six numbers representing these intervals 

 were arranged according to their proper dates and it was noticed that 

 they could be made to fit in with a periodic change of about 35 years in 

 length. Again, the area of the sunspot curve from one minimum to 

 the next was found, and another set of six numbers was obtained, the 

 last five of which would again fit into a 35-year period. The two 

 series corresponded well with each other if we excepted the first of 

 each, the reason for the exception being that there was a doubt as to 

 the observations from which the first number of the second set is ob- 

 tained having been made on the same plan as the others. Unfor- 

 tunately, the evidence thus presented is far from conclusive. There is 

 nothing unnatural in the two series agreeing with one another as far as 

 a period is concerned since they were deduced from the same set of 

 observations. The difficulty arises in the attempt to find the length 

 of the period from five or six numbers spread over rather less than 

 two revolutions of the cycle. One would prefer to say that this result 

 was a reason for further investigation rather than that it proved any- 

 thing definite as to the existence of a 35-year period. 



Far more numerous are the difficulties which beset an examination 

 of the conditions on the earth's surface. In the first place, accurate 

 observations are practically confined to the latter half of the last cen- 

 tury, and these have been made chiefly in the northern temperate zones 

 where the daily and weekly changes are apt to be very irregular and 

 violent and where the local conditions frequently exercise much in- 

 fluence in determining the weather or climate of a particular place. 

 To obtain averages free from these local and temporary conditions re- 

 quires the examination of a very large number of observations ex- 

 tended over a long period. In the second place, the observations at 

 one place should be kept separate from those at other places, for it is 

 theoretically possible and even probable that a maximum at one place 

 of observation may occur at the same time as a minimum at another 

 place. For example, the yearly averages might show that a maximum 

 rainfall in one place always occurred with a minimum rainfall in 

 another, and vice versa. If the results from the two places were com- 

 bined, a part or the whole of the periodic change would be lost. Then 

 again, there is no clear indication what observations should be chosen 

 for examination; average daily temperature, maximum or minimum 

 daily temperatures, rainfall, number of violent storms, number of days 



