SECTIONAL TRANSACTIONS.—A. 339 
the Yerkes Actinometry, I (Parkhurst); the Yerkes Actinometry, II (Fairley); and 
the Leiden magnitudes (A. de Sitter). 
The visual magnitudes of 1,300 of the stars in the catalogue have been determined 
at Harvard in the various Harvard Photometries. With the aid of the photovisual 
standards recently set up in the Harvard Standard Regions (Harvard Annals, 89, No. 1) 
these visual magnitudes have been rendered homogeneous, and they are used to obtain 
the colour indices given in the catalogue. The revised visual magnitudes are very 
close to the Potsdam visual scale. , 
The spectral classes of all stars in the catalogue taken from the Henry Draper 
Catalogue are used in examining the dispersion of colour within one spectral class. 
Miss Ince Lenmann.—On the Construction of Seismic Time-curves for 
great Distances. 
In earthquake diagrams recorded at great distances from the epicentre various 
phases not found at smaller distances occur. The diagrams are more complex and 
special methods are required for the determination of time-curves. 
The data of the bulletins are available for the determination of time-curves for 
P and § for distances up to about 85°. With respect to the phases recorded at greater 
_ distances the bulletins do not give sufficient information. Most stations do not 
publish detailed readings of their seismograms, and even if this were done it would 
hardly yield the data required for a satisfactory determination of time-curves. The 
diagrams recorded at great distances are often difficult to read ; some of the phases 
follow each other closely, and often the movement, though strong and varying in 
_ appearance, does not contain clearly marked phases; the reading of the diagrams 
_ of a single station, therefore, gives an uncertain result. 
Tf, however, records from many stations which are not very far apart can be 
brought together and read by one observer a satisfactory reading can be obtained. 
When the records are compared with one another their characteristics become apparent 
and the phases which repeat themselves can be traced in all. If for an interval of 
distance a sufficient number of records are available, the course of the time-curves 
can be constructed and the phases can be identified. y 
; For the construction of time-curves a determination of epicentre is required, and 
this as a rule cannot be very accurately made. But, if we use the records of a group 
of stations at a great distance from the epicentre, and if we are satisfied with deter- 
mining the course of the time-curve and do not require the time-curve itself, no great 
_ accuracy is needed in the determination of the epicentre. For, if the epicentre adopted 
is shifted to a slightly different position, all the distances to the observing stations 
are altered by very nearly the same amount, and the course of the time-curve remains 
thesame. Only the course of the time-curve is needed for the identification of a phase. 
For five earthquakes the records of the European group of stations have been 
studied. The phases have been traced, the course of their time-curves determined 
and the phases identified. The results which have been obtained have been compared 
_ with one another and differences between the time-curves of the different earthquakes 
have been found. The ratio PP: P has also been considered, and has been found to 
differ in different cases. 
_ The phase P’ was studied from the records of the New Zealand earthquake of 
16V11929.! It was found that, for the distances of the European stations, ¢. 160-170°, 
phases were recorded corresponding to two branches of the time-curve of P’. The 
time-curves of such other phases as could be traced in all the records available were 
o determined,’ and compared with those constructed by B. Gutenberg. 
_ The following earthquakes were studied for the range of distance mentioned, viz. : 
22 IIT 1928, 4» (80-90°)? ; 18 VIL 1928, 19» (85-95°); 13 XI 1925, 126 (938-100°) ; 
24 X 1930, 20» (95-105°). 
_ 11]. Lehmann: P’ as read from the records of the earthquake of June 16, 1929. 
ferl. Beitr. Geoph. 26, 1930. 
_ * 7]. Lehmann: Die Bedeutung der europiischen Stationsgruppe fiir die Bestim- 
mung yon seismischen Laufzeitkurven. Verhandl. fiinflen Tagung. Balt. Geod. 
Komm. Helsinki, 1931. 
_ * J. Lehmann: The earthquake of 22 IIT 1928. Gerl. Beitr. Geoph. 28, 1930. 
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