270 



NATURE 



\ytdy 21, 1887 



centering at Washington during the week following the 

 earthquake ; (5) to geologists and weather bureaus of 

 several States, who have kindly exerted themselves in this 

 matter and collected much important information ; (6) to 

 a considerable number of scientific gentlemen who have 

 distributed for us our circular letters of inquiry in special 

 districts,— notably, Profs. W. M. Davis, C. G, Rockwood, 

 J. P. Lesley, T. C. Mendenhall, and Messrs. W. R. Barnes, 

 of Kentucky, and Earle Sloan, of South Carolina ; (7) to 

 a large number of postmasters in the Eastern, Central, and 

 Southern States; and, finally, to hundreds of miscellaneous 

 correspondents throughout the country. 



In collecting this information, a printed list of questions 

 was prepared. This practice has been resorted to in 

 Europe and in Japan with considerable success, and the 

 questions which have been devised for distribution in 

 those countries have been prepared with great skill by 

 some of the ablest investigators of earthquakes. Prof. 

 C. G. Rockwood, of Princeton, has also been in the habit 

 of distributing formal questions of this character in this 

 country whenever apprised by the newspapers of a notable 

 shock. Availing ourselves of his advice and assistance, 

 questions prepared by him were printed and widely dis- 

 tributed. They were much fewer and more simple than 

 those employed in Europe, because European investi- 

 gators depend almost wholly upon the educated classes to 

 answer them, while in this country the uneducated but 

 intelligent and practical classes of the people must be the 

 main reliance. These questions were designed to elicit 

 information : (i) as to whether the earthquake was felt, 

 (2) the time of its occurrence, (3) how long it continued, 

 (4) whether accompanied by sounds, (5) the number of 

 shocks, (6) general characteristics which would serve as 

 a measure of its intensity and indicate the kind and 

 direction of motion. 



It is to be observed that the only information to be 

 hoped for which can have even a roughly approximate 

 accuracy is the time of transit of the shock. The degree 

 of approximation in the time data actually obtained will 

 be adverted to later. Special effort was made to obtain 

 information as to the relative intensity of the shocks in 

 all parts of the country. At the very outset a serious 

 difficulty presents itself. In the estimates of intensities 

 there is no absolute measure. What is really desired is 

 some reliable indication which shall serve as a measure of 

 the amount of energy in any given portion of the wave 

 of disturbance as it passes each locality. The means of 

 reaching even a provisional judgment are very indirect, 

 and qualified by a considerable amount of uncertainty. 

 To estimate the force of a shock, we have no better means 

 than by examining its effects upon buildings, upon the 

 soil, upon all kinds of loose objects, and upon the fears, 

 actions, and sensations of people who feel it. In view of 

 the precise methods which modern science brings to bear 

 upon other lines of physical research, all this seems crude 

 and barbarous to the last degree. But we have no other 

 resource. Even if it were possible to obtain strictly com- 

 parative results from such facts, and decide with confidence 

 the relative measure of intensity which should be assigned 

 to each locality, we should have gained measures only of 

 a series of local surface intensities and not of the real 

 energy of the deeply-seated wave which is the proximate 

 cause of the surface phenomena. Notwithstanding the 

 indirect bearing of the facts upon the real quantities we 

 seek to ascertain, and their apparently confused and dis- 

 tantly related character, they give better results than 

 might have been supposed. When taken in large groups, 

 they give some broad indications of a highly suggestive 

 character, and though affected with great inequalities 

 which for the time being seem to be anomalous, these 

 anomalies are as instructive as the main facts themselves. 

 We have given the preliminary plotting of the intensities 

 in the map before you. The first point to which we shall 

 invite attention is the magnitude of the area affected by 



the shocks. It was sensibly felt in Boston, which is the 

 most distant point on the Atlantic coast from which 

 affirmative reports have been received. From Maine the 

 answers are all negative. Most of those from New 

 Hampshire are negative, but two or three positive ones 

 show clearly that it was felt in sensitive spots. In 

 Vermont, affirmative reports come from St. Johnsbury 

 and Burlington on Lake Champlain. No positive reports 

 come from the province of Quebec. In New York State 

 it was felt in the vicinity of Lake George, and at Lake 

 Placid and Blue Mountain Lake in the Adirondacks. In 

 Ontario, it was quite noticeable in several localities, 

 though the great majority of reports from that place are 

 negative. In Michigan, it was noted in several places, 

 and at Manistee Lighthouse, on Lake Michigan, the 

 trembling was strongly marked. In Winconsin, though 

 most of the reports are negative, it was felt quite strongly 

 at Milwaukee, and was also noticed at Green Bay, and at 

 La Crosse on the Mississippi, 967 miles from Charleston, 

 the remotest point in the United States which gives a 

 positive answer. In Central Iowa and Central Missouri, 

 it was unmistakably felt. In Arkansas, the eastern portion 

 of the State, from sixty to seventy-five miles west of the 

 Mississippi, gives numerous favourable reports. In 

 Louisiana, the reports are mostly negative, but numerous 

 persons in New Orleans felt the shocks and recognized 

 their nature. In Florida, it was universally felt, and in 

 the northern part of the State was severe and alarming. 

 From the Everglade region, of course, no reports have 

 been received, as it is uninhabited ; but in some of the 

 Florida Keys it was felt in notable force. From Cuba a 

 few reports have cone, and the most distant point in that 

 island which was shaken was Sagua la Grande, where the 

 vibration was very decided. Lastly, a report comes from 

 Bermuda, 1000 miles distant from Charleston, which leaves 

 little doubt that the tremors were sensible there. 



The area within which the motion was sufficient to 

 attract the attention of the unexpectant observer would 

 be somewhat more than circumscribed by a circle of 1000 

 miles' radius, and the area of markedly sensible shaking 

 would, including the oceanic area, be somewhere between 

 two and a half and three million square miles. In this 

 estimate, however, only well-defined seismic movement of 

 notable force is considered. There are reasons for 

 believing that by proper instrumental observation the 

 movement could have been detected over a much greater 

 area. In the first place it is to be noted that the peripheral 

 portions of the observed area lie in districts which are 

 rather thinly populated, sometimes, also, in districts which 

 from the nature of the ground do not disclose forcibly 

 the passing shock. Furthermore, the passing wave in the 

 outer portions of the area was almost everywhere of an 

 undulatory character and of great wave-length, and while 

 still retaining a large amount of energy, did not often 

 dissipate itself into those smaller and shorter tremors 

 which are very much more likely to attract attention, 

 though really possessing very much less energy. Six 

 hundred miles from the origin the long swaying motion 

 was felt, and was often sufficient to produce sea-sickness, - 

 yet was unaccompanied by sound or by the tremulous 

 motion due to short waves. 



It will be observed upon the map that there are several 

 large tracts which show a comparatively feeble intensity, 

 while completely surrounding them is the general area of 

 greater intensity. The most conspicuous of these areas 

 of silence is the Appalachian region. The facts here are -^ 

 extremely interesting and suggestive. It has been gener- " 

 ally supposed that a mountain-range serves as a barrier 

 to the propagation of earthquakes— not from any known 

 relation of cause and effect, but merely as the result of 

 observation. In Japan it is universal testimony that the 

 central range of the island marks the dividing line between 

 earthquake and no earthquake. The shocks so frequent 

 there are seldom or never felt beyond the mountains. A 



