September 10, 1886.] 



SCIENCE. 



225 



days previously moderate shocks were felt near 

 Charleston. From the Carolinas it radiated with 

 great rapidity (from 20 to 60 miles a minute) 

 throughout the great area bounded on the south 

 by the Gulf of Mexico ; on the north by Michigan, 

 the province of Ontario, New York, and southern 

 New England ; on the east by the Atlantic ocean, 

 where it was probably felt nearly 500 miles at sea ; 

 and on the west by the central Mississippi valley. 

 The limits are, so far as now known, as follows : 

 central Florida ; eastern Louisiana, Arkansas, Mis- 

 souri, and Iowa ; southern Michigan and province 

 of Ontario ; northern New York ; and southern 

 New England. It was not felt at Bermuda. The 

 limits of the shock, as here stated and as indicated 

 in the accompanying map, it is particularly de- 

 sirable to verify, as well as the correct time at 

 which the shock was first felt at all points within 

 the disturbed area. It often happens that there 

 are places within an earthquake area where the 

 shock is not perceptible, owing probably to sonae 

 local peculiarity in the geological formation, 

 although decidedly noticeable at places not far 

 away. There are already points of this kind men- 

 tioned, — in Florida, Indiana, and Connecticut, 

 for instance, — and such information is very in- 

 teresting. 



The hypothesis has been advanced by Perrey 

 that earthquakes are connected with subterranean 

 tides due to the combined influence of the sun 

 and moon, and analogous to those in the ocean. 

 At a given point the earth's strata are under the 

 accumulated tension of centuries, and this press- 

 ure is slowly but steadily increasing, until it 

 reaches a point when fracture is imminent. 

 Twice a day the great oceanic tidal waves sweep 

 along the coast, the tremendous changes of press- 

 ure due to them being possibly augmented by 

 analogous movements beneath the crust ; and at a 

 critical moment they add ' the last straw ' that 

 determines the fracture. It is very interesting to 

 notice in this connection that at the time of the 

 severe shock at Charleston this tidal influence was 

 at its maximum. The moon was in perigee at 

 2 A.M., Aug. 29 ; new moon at 8 a.m. the same 

 day, acting in a direct line with the sun (the 

 eclipse of the sun occurred at 5 a.m., Aug. 29) : 

 extremely high tides occurred, therefore, for sev- 

 eral days following. The moon's upper transit at 

 Charleston occurred at 2.22 p.m., on Aug. 31. The 

 high tide following (the higher of the two daily 

 tides) was at 9.35 p.m., just twenty minutes before 

 the shock occurred. This remarkable coLucidence 

 is of course extremely interesting. 



It seems remarkable that no sea-wave followed 

 the shock ; and indeed it was providential that it 

 did not, as the resulting destruction and loss of life 



would have been a hundredfold greater. A sea- 

 wave (often very incorrectly called a tidal wave) 

 of greater or less size is the almost invariable ac- 

 companiment of a severe shock occurring near 

 the seacoast. 



It is unnecessary to enlarge here and now upon 

 the general effects of this severe earthquake, or to 

 theorize upon the causes of earthquakes in general 

 or of this one in particular, more than has aheady 

 been done. Such a study, to be of any value, 

 must await the compilation and elaboration of a 

 vast amount of material, and the final reports of 

 the geologists who are now at work in the region 

 of greatest disturbance. 



STUDY OF THE EARTHQUAKE. 



The U. S. geological survey has undertaken to 

 make a study of the severe earthquake of Aug. 31, 

 which caused such great destruction and loss of 

 life at Charleston, S.C. It was the most severe on 

 record in the United States, both as to the effects 

 produced and the area disturbed. 



The study of phenomena of this kind is of the 

 greatest value to science as a guide to the knowl- 

 edge of the nature of the interior of the globe, and 

 in its bearing upon every branch of physics and 

 geology. In it there is needed a vast amount of 

 reliable information, not only from points within 

 the disturbed area, but also from adjacent points, 

 in order to accm-ately define its limits ; and it is 

 not only skilled observers who can furnish such 

 information, but almost every one can contribute 

 valuable facts. It is therefore confidently hoped 

 that facts of interest will be sent in at once to the 

 U. S. geological survey at Washington while they 

 are still fresh in the memory. Newspapers can 

 render great assistance by giving wide publicity 

 to this call, and by sending copies of their issui^s 

 containing information about the local effects Oi 

 the shock. Attention to the points mentioned 

 below will add greatly to the value of the in- 

 formation, and facilitate its elaboration and 

 study. 



Write on one side only of the paper. After dating 

 the letter as usual (giving also the locality where 

 the observation was made, if not the same), write 

 ' Answers to circular No. 2.' State the observer's 

 situation (whether in the house or out of doors, up 

 stairs or down, sitting, standing, walking, reading, 

 etc.) ; also, if possible, the character of the ground 

 (whether rocky, earthy, sandy, etc.) Then an- 

 swer the following questions, referring to them by 

 number only : — 



1. Was an earthqiiake felt at your place the even- 

 ing of Aug. 31, 1886, or within a few days of that 

 time ? Negative answers to this will be of great 



