CLAYTON. — PERIODICITY IN THE WEATHER. 



613 



TABLE IL 

 Departures of Tempkrature prom Normal. 



were charted on a map and lines of equal' departure drawn. These lines 

 are shown in Plates VII. and VIII. The continuous lines show plus 

 departures and the broken lines show minus departures. The results 

 show that there were distinct centres of oscillation which had a move- 

 ment in space. From this it follows that the phase of the period at any 

 one point on the earth's surface will reverse, and the range and form of 

 the cycle cannot be found from averages of observations taken at one 

 point during a great many periods. 



The next period which attracted my attention were oscillations in the 

 temperature of about thirty and of about seven days. The results of these 

 investigations were published in the American Meteorological Journal for 

 June and August, 1895. The seven-day period was found to be about 

 one fourth the length of the thirty-day period, and hence was assumed 

 to be a harmonic of the latter. (American Journal of Science, March, 

 1894.) By grouping the phenomena into periods of three years, I found 

 a certain constancy in the average position of the maxima and minima 

 of the seven-day period, and ascertained the length of the period as 

 7 days G.4 hours. In 1894, with the assistance of Mr. Sweetland, I 

 ascertained the range of all the periods which were multiples of this 

 length up to fifty-eight days. It was found that the period of about 

 twenty-nine days gave the greatest range, and this led me to consider 

 whether the synodic rotation of the moon was concerned in the matter. 

 It was found that the length of my seven-day period would be of the 

 same length as one quai'ter of the synodic rotation of the moon if the 

 number of periods had been assumed one less in the three-year groups in 

 which I had arranged the phenomena. It followed that the lunar period 



