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 Wolf's 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 312 years to about 412 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 obtained having been made on the same plan as the others. Unfortunately, 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 anything 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 century, 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 influence in determining the weather or climate of a particular place. To obtain averages free from these local and temporary conditions requires the examination of a very large number of observations extended 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 combined, 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
