Page:Popular Science Monthly Volume 66.djvu/512

508 the earliest times, the fact that they undergo periodic changes in number and size was not recognized until long after the discovery of the telescope, and the law of the change was not determined with anything like accuracy until the middle of the last century. Accurate observations of their number and size were only commenced about 1830, but from about 1750 to 1850 enough material had been gathered to show that a maximum occurred on the average every eleven years. In the last two or three decades the use of the spectroscope has added greatly to our knowledge of their nature and motions, and the photographic camera has enabled astronomers to add permanent records of the state of the sun's surface at any time to the numerical estimates which were the chief contribution of the earlier observers. Such pictures are of special value whenever, as in the case of sunspots, much depends on the personal equation of the observer and still more on the particular method used in forming an estimate of them. Thus there is a period extending over the last seventy years in which continuous observations have been made, a period of another seventy years earlier in which the observations are worth discussing, although much less reliance can be placed on them for accurate deductions, and a still earlier period of about a hundred years from which a little doubtful information can be gleaned.

The net result which has been deduced from this series is an average period of 11 years between the maxima or times of greatest sunspot activity. But this average period by no means represents all the facts. The time between two consecutive maxima has been as long as 17 years and as short as 8 years, while some maxima are marked by much larger and more numerous spots than others. The average period between the minima or dates of fewest sunspots is rather less irregular, since it has never been known to differ by much more than two years from the mean period of 11 years. There is some evidence that these stronger and weaker maxima themselves run in a cycle, but the period is very doubtful. Wolf, who devoted most of his life to the investigation of sunspots, deduced a period of 55 years for this longer cycle, while there are later determinations of 60, 35 and 61 years. Further, the curves representing the number of spots from day to day or from month to month show many other irregularities which have up to now defied any attempt to group them in any regular order. Thus we have a well-marked average period of about 11 years and doubtful ones of about 35 and 60 years. These are therefore the periods into which the meteorological records are to be grouped in order to inquire whether there is any connection between the two sets of phenomena.

There is good reason to believe that a period of about 35 years is to be traced in certain of the meteorological records, and therefore it is