Astronomy and Seismology. 155 



track between these points was 29 # 3 miles. The velocity with 

 which a Perseicl enters the air is 37*2 miles per second ; hence the 

 duration of the flight was probably less than one second. 



On the following evening plates were exposed without success. 

 Everything was in readiness on the 15th and 16th of November, 

 in case the Leonids should appear in sufficient numbers to justify 

 the exposure of plates. The moonlight and clouds interfered 

 on the 23d to 27th of November, when the Bielids might possibly 

 appear in moderate numbers. 



On the evening of December 10th, the work was again resumed, 

 and several plates were exposed, two cameras being used. They 

 were directed near to the Geminid radiant, and three tracks of 

 true Geminids were secured. Mr. Lewis at Ansonia joined in the 

 work, but upon his plates only one possible trace was printed. 

 This is too doubtful to be safely used. 



The three Observatory tracks meet very nearly in a point. In 

 fact a provisional computation based upon the Bonn charts, and 

 allowing for differences of zenith attraction of the meteors makes 

 the circle inscribed in the small triangle formed by the tracks 

 produced backward less than one minute in diameter. The Bonn 

 Durchmusterung star-places do not justify this degree of accuracy. 



The experiment shows that with a reasonable amount of labor 

 we can get photographs of meteor tracks. The accuracy of results 

 obtained from the photographs is unspeakably greater than those 

 gotten by any other known method. h. a. n. 



3. On the Annual and Semi-annual Seismic Periods; by 

 Charles Davison, (from the Proceedings of the Royal Society, 

 vol. liv, pp. 82-85.) — (Abstract). Method of Investigation. — The 

 method adopted is similar to that employed by Dr. C. G. Knott 

 in his paper on "Earthquake Frequency." 



A definition of the unit earthquake having been adopted, the 

 earthquakes of different districts are classified in halt-monthly 

 groups, the first half of February containing fourteen days, and 

 of all the other months fifteen days ; and the numbers so ob- 

 tained are reduced to intervals of equal length (fifteen days). 

 The numbers for the two halves of each month are added 

 together. The mean of the numbers for the six months from 

 November to April gives the six-monthly mean corresponding to 

 the end of January. Six-monthly means are calculated in this 

 way for the end of each month; each mean is divided by the 

 average of all twelve, and the difference between each quotient 

 and unity is multiplied by the augmenting factor 1*589, in order 

 to obtain the correct value of the ratio a^ : a . The curve ob- 

 tained by plotting these reduced means thus gives special pro- 

 minence to the annual period, by eliminating the semi-annual 

 period and all those which are fractions of six months, and by 

 diminishing the amplitudes of all other periods with respect to 

 that of the annual period. 



In investigating the semi-annual period, the numbers corres- 

 ponding to the first halves of January and July are added to- 



