ON EARTH TREMORS. 



319 



The curves representing these daily changes bear a resemblance 

 to the curves of the daily changes of temperature and of magnetic decli- 

 nation. The ordinates for every two hours are as follows : — 



As was pointed out before, the daily motion of the horizontal pendu- 

 lum is extremely variable, especially as far as the range of motion is 

 concerned. Whilst the epochs of elongation do not differ much from 

 the mean values within a small number of days, they appear to be subject 

 to an annual change. But owing to the observations being incomplete 

 the character of this annaal variation is not sufficiently well-determined. 

 Thus, at Orotava, during four months of observations in winter, we find 

 that in the earlier part (December) the maximum deflections occur aboat 

 two hours later than towards the end (April). At Wilhelmshaven the 

 eastern elongation is earlier in spring and autumn than in midsummer, 

 whilst at Potsdam there is no very marked change. 



The range of motion (a) was found to be much more variable, and it 

 was interesting to compare it with the maximum oscillation of tempera- 

 ture (^) during the day as well as with the observations on sunshine 

 (S) and clouds (CI). The following mean results were obtained : — 



Celsius (Centigrade). 



2 Clear sky = 0, clouded sky = 10. 



