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XX. — An Investigation of the Seiches of Loch Earn by the 

 Scottish Lake Survey. 



Part III. : Observations to Determine the Periods and Nodes. — Part IV. : Effect of 

 Meteorological Conditions upon the Denivellation of Lakes. — Part V. : Mathe- 

 matical Appendix on the Effect of Pressure Disturbances upon the Seiches in 

 a Symmetric Parabolic Lake. By Professor Chrystal. 



(MS. received July 2, 1908. Read November 16, 1908. Issued separately November 24, 1908.) 



PART III. 

 OBSERVATIONS TO DETERMINE THE PERIODS AND NODES. 



Determination of the Periods. 



In the first part of the present series of communications # I have given a general 

 discussion of the apparatus and methods of observation employed in the investigation 

 of the seiches of Loch Earn, and have also mentioned some of the errors to which seiche 

 observations are liable, and how they may be avoided or corrected. It may be useful 

 to enumerate here certain of these errors, omitting for the most part those that arise 

 from the construction of the instruments, and can be eliminated by proper design and 

 by preliminary tests. We have the following, arranged more or less in what experience 

 seems to show to be the order of their ultimate importance : — 



1. Errors arising from irregularities in the rate of the driving clocks of the fixed 

 limnographs. 



2. Errors due to the hygroscopic expansion and contraction of the registering paper 

 in the fixed limnographs. 



3. Changes of phase, sudden or gradual, due to disturbances generating seiches of 

 the same period as the one under observation. 



4. Irregular displacement or blurring of the turning-points on the limnogram by 

 wind disturbances, trains of surface waves, or seiches of very short period, 



5. Uncertainty of the turning-points owing to irregular frictional lagging of the 

 axle in the index limnographs, and also owing to the discontinuity in the reading of 

 these instruments. 



G. Displacement of the turning-points by the interference of seiches of different 

 nodality. 



7. Errors of the observers' watches. 



* Trans. R.S.E., vol. xlv.,p. 362, hereafter quoted as I.S.E, 

 TRANS. ROY. SOC. EDIN., VOL. XLVI. PART III. (NO. 20). 70 



