CHAPTER IX. 
SEISMIC EFFECTS OTHER THAN THOSE DUE TO EARTHQUAKES. 
Dr. MILNE once remarked to me that a seismogram always 
has something to show worth knowing even if there is no 
earthquake. Those who have had the great privilege of visit- 
ing the observatory at Shide and seeing Dr. Milne’s wonderful 
album of seismograms will appreciate how true the remark is, 
and how thoroughly Milne has devoted himself to anything 
that can throw light on the subject which he has so conspicu- 
ously adorned. 
We must pass over the spurious effects on a seismograph 
produced by the presence of the observer, the shutting of doors, 
and that bane of the experimentalist, the ubiquitous spider. 
They are mentioned here, only to point out that the practical 
seismologist must be able to recognize such effects when they 
occur. 
It was long ago recognized by Milne that a seismograph 
frequently shows minute vibrations continuing for many hours. 
and that they could not be accounted for by earthquakes or 
local traffic. 
These effects were called by him ‘‘ Tremors” and although 
they occur always with high local winds, they also appear 
when it is quite calm. 
On the Milne seismograms the tremors present the same 
appearance on calm or on gusty days. But with heavily 
damped seismographs, using larger magnification and higher 
speed of registration, it is found that there are two types of 
tremors or microseisms as they are now called. They are 
shown to special advantage on Galitzin seismograms. 
In the first class, which occurs on windy and calm days 
alike, the movements are very smooth and regular, and the 
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