ON SEISMOLOGICAL INVESTIGATIONS. 93 
VII. Note on the Duration of the First Preliminary Tremor in the San 
Francisco and Colombian Earthquakes. By R. D. OLpuam. 
The great earthquakes of Colombia, January 31, 1906, and California, 
April 18, 1906, originated at not very greatly different distances from 
Western Europe, but reached it by very different wave-paths. The great 
circles from Shide, plotted on the map, show that in the former case the 
wave-paths lay under the broadest and deepest part of the Atlantic Ocean, 
and in the latter under the continent of North America and the continental 
shelf of the North Atlantic. In studying the records of these two earth- 
quakes I found that there was a marked difference in the interval 
between the arrival of the first and the second phases of the preliminary 
tremors, the interval at Shide being 9:9 minutes for a distance of about 
77°6 degrees in the Californian, and 11°9 m. for a distance of about 80:7 
degrees in the Colombian, records. The difference between these two, 
viz., 2-0 minutes, is greater than the average, but a comparison of all the 
records from European observatories, at which these two phases can be 
recognised in the case of both earthquakes, gives mean intervals of 10-4 
and 11:4 minutes for mean distances of 84° and 86° in the case of the 
Californian and Colombian earthquakes respectively. The difference 
in interval, corresponding with the difference in distance, is only 
0-2 minute, or one-fifth of the observed difference. As these two phases of 
the record are due to wave-motion of different kinds, transmitted at dif- 
ferent rates, through the earth, the difference in interval between their 
arrival indicates a difference in the ratio between their rates of trans- 
mission, and consequently a difference in the constitution of the matter 
under the North American continent and the North Atlantic Ocean. 
The exact time of occurrence of the Colombian earthquake being un- 
known from direct observation, it is not possible to compare the absolute 
rates of propagation and determine in what this difference consists, but 
it appears to be too great to be due to any error of record or interpreta- 
tion, and may be accepted as real. 
Magnetic Observations at Falmouth Observatory—Report of the 
Committee, consisting of Sir W. H. PREECE (Chairman), Dr. R. T. 
GLAZEBROOK (Secretary), Professor W. G. Apams, Dr. CHREE, 
Captain Creak, Mr. W. L. Fox, Sir A. W. RicKer, and Professor 
SCHUSTER. 
THE grant voted by the Association last year has been expended in 
maintaining the magnetic observations at Falmouth Observatory. The 
results of the observations have been published in the Annual Report of 
the National Physical Laboratory, as well as in that of the Royal Cornwall 
Polytechnic Society. 
The mean value of the magnetic elements for the year 1906 are— 
Declination . : MOS soWe 
Horizontal Force . . 018790 C.G.8. 
Vertical Force : . 043344 C.GS. 
Inclination . 3 BEGG oHoo Te UN 
The Committee are informed that the observatory at Eskdale Muir 
will be ready for occupation this autumn. It is clearly important for 
the sake of continuity that the Falmouth Observatory should be fully 
maintained until Eskdale Muir is in complete working order. To secure 
this the Committee ask for reappointment, with a grant of 50/. 
