6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 98 



annual period in temperature at New York is approximately constant 

 in phase from year to year and nearly constant in amplitude. 



If next a trial for shorter periods is made, it is found that when 

 a cosine series of 24 terms is multiplied by the observed hourly 

 temperatures, a period of 24 hours stands out prominently. Figure 3 

 shows the daily period in temperature obtained from hourly observa- 

 tions of temperature at Blue Hill Observatory, near Boston, Mass. 

 This plot shows that the maxima and also the minima of temperature 



Fig. 2. — Monthly temperatures at New York, smoothed harmonically. 



Fig. 3. — Hourly temperatures at Blue Hill, smoothed harmonically. 



occur near the same hours every day, but that the period is very 

 variable in amplitude. Further research shows that this period occurs 

 only in the lower atmosphere and that the oscillations are large when 

 the sky is clear and small when there are dense clouds. Hence, its 

 variability in amplitude is closely related to moisture and cloudiness. 



Yearly means of atmospheric pressure, when treated in the same 

 manner, show clearly defined oscillations of 10 to 12 years. In figure 4, 

 the result of analyzing the pressure at Stykkisholm, Iceland, with a 

 12-term series is shown by a continuous line and is compared with a 

 plot of yearly sunspot numbers shown by a dotted line. 



The sunspot curve shows a fairly regular period which averages 

 about 1 1 years, but varies in length and amplitude. 



