ACADEMY OF NATURAL SCIENCES. 151 



localities the motion was undulatory. The shock was not felt at Monterey, 

 twelve miles west of San Juan. 



June. — A smart shock at San Francisco. 



July 15th, lOh. 19ra. — A smart shock at San Francisco. 



August 1st, llh. 6m. — Two light shocks at San Francisco about one hour 

 apart. 



December 19th, 12h. 38m. — A very smart shock was felt throughout the city ; 

 directly afterwards another and more severe one occurred. The first was a 

 sharp, sudden jar, the second undulatory. The accuracy of the telegraph 

 operator at Santa Clara enables us to form a correct idea of the course of this 

 wave. His time was 14h. 44m. 31s., and within twenty-nine seconds of true 

 time. The elapsed time is 7m. 31s., and gives for the direction of the seis- 

 mic wave a course north and south (in lieu of east and west in my first notice 

 which was thus in error.) I take this opportunity to express the thanks of 

 the Academy to this operator for his accuracy and kindness in furnishing us 

 dates in this and other phenomena of scientific and public interest. 

 The figures derived from our statistics furnish us the following interesting 



results as to the frequency of shocks in one season of the year more than in 



another. The tables below furnish the details. 



It is found from these figures that during the thirteen years ending December, 



1863: 



First. The number of days on which earthquakes have occurred, is one hun- 

 dred and ten. 



Second. The month in which the greatest number have occurred is January, 

 being sixteen ; and the months in which the least number have occurred, is 

 April and February, the sum of each being six. 



Third. The winter months have given the largest number in the aggregate, 

 the sum being thirty-four. The summer months the smallest number ; their sum 

 being twenty-three. The spring months have given twenty-four, and the autumn 

 months twenty-nine. 



Fourth. The number of shocks between the equinoxes foot up thus. Between 

 the autumnal and vernal equinoxes the sum is sixty-four ; between the vernal 

 and autumnal equinoxes, the sum fifty-two. 



Fifth. The number of shocks that have taken place between the solstices 

 stand thus. From the winter to the summer solstice the sum is fifty-four. From 

 the summer to the winter solstice, the sum is fifty-nine. 



