190 



KNOWLEDGE. 



[August 1, 189C. 



leadiDg ; it would be better far if such an observation had 

 never been made. As a general rule, therefore, when an 

 observer is unprovidid with a really good watch, ho will 

 spend his time more usefully in giving his whole attention 

 to the nature of the shock and the accompanying sound. 

 When the earthquake is over he should then record the 

 time, and it is not dilllcult to estinuito the brief interval 

 which has elapsed. When, as is often tlio case in this 

 country, successive earthquakes are separated by months 

 or years, an error of a minute or two is not of much con- 

 sequence. The time, however, should always be recorded 

 as nearly as possible, if only for the purpose of identifying 

 the shock with that observed by others. Sometimes also 

 it is useful in tracing a doubtful or reported earthquake to 

 its real origin, such as the firing of heavy guns at a 

 distance, or the bursting of a meteor. 



The preceding remarks apply in part to the duration of 

 the shock. There is no difficulty, of course, in determining 

 this with some accuracy ; but here again the time can be 

 more usefully employed. The duration is an element of 

 little importance, unless it is determined by a seismograph, 

 because the instants at which the movement begins and 

 ceases to be perceptible depend entirely on the sensitive- 

 ness of the observer ; and this varies in different persons, 

 and even in the same person at different times. For all 

 practical purposes, the direction can be estimated with 

 sufficient accuracy when the earthquake is over. The 

 observer should place himself in the same position as that 

 in which he felt the shock and then imagine it repeated, 

 marking the beginning and end, the aid of another person 

 being obtained to time the interval. 



One of the most important points to which an observer 

 can direct his attention is the intensity of the shock. If 

 nothing else but the mere occurrence of the sound and 

 shock were noted, this element should not be omitted. 

 Fortunately we are provided with a rough scale of seismic 

 intensity, which has met with general adoption in nearly 

 all covmtries where earthquakes are studied, and which is 

 accurate enough for most purposes, besides being most 

 easy to apply. This is known as the " Rossi-Forel " scale 

 of seismic intensity." The different parts of Question G 

 in the list at the end of this paper refer to degrees iv. to 

 viii. of this scale, the degree being added in brackets after 

 each part of the question. One or two remarks should 

 perhaps be made on this subject. In his own notes, at any 

 rate, the observer should record a full answer (not merely 

 "Yes") to each part of the question when it can be so 

 answered. It should, for instance, be stated whether, in 

 answer to the first part, doors, windows, etc., all rattled, 

 or only one or the other ; and whether the rattling was 

 violent or slight. Answers, again, should be written to 

 all parts of this question, especially if in the negative. 

 The last affirmative answer of course determines the 

 intensity of the shock. On this subject of intensity, 

 however, the observer should not rest satisfied with his 

 own impressions. lie should make inquiries of his 

 neighbours and others situated at a short distance in the 

 same town or village, for the intensity often differs con- 

 siderably at two points which are not very far apart. f 



The last subject which need be referred to here is the 

 observation of the sound phenomena. These, as a rule, 

 are somewhat neglected, and yet they may prove to be of 

 considerable importance, especially in concert with a large 

 number of similar observations made elsewhere. The 



* For a full translation of the scale, tee Nature, Vol. XLII., 1690 

 p. 349. 



t Ste a paper by Prof. J. Milne, "On a Seismic Survey made in 

 Tokio in 188-1 and 1885." Japan Seismol. Soc. Trcns' Vcl. X., 

 1887, pp. 1—36. 



earthquake sound should of course be carefully distinguished 

 from that produced by the rattling of loose objects, or, in 

 rare cases in this country, by the fall of masonry. There 

 is, indeed, little risk of confusion, for the earthquake sound 

 does not resemble very closely any ordinary noise. On 

 this account it is somewhat difficult to describe ; but it is 

 often compared to thunder (especially the low roll of 

 distant thunder), the passage of a traction-engine or 

 heavy vehicle along a rough road, the dragging of fur- 

 niture across the floor of a room overhead, the roaring of 

 a chimney on fire, or the rush of a gust of wind. Some- 

 times it is more or less short and abrupt, like the firing of 

 distant cannon, the fall of a heavy body, or the slamming 

 of a door ; and occasionally it resembles a succession of 

 these, such as would be produced by a hard and heavy 

 ball rolling down a short flight of stairs. Particular 

 attention should be paid to the time relations of the 

 shock and sound- — whether the beginning of the sound 

 precedes, coincidas with, or follows the beginning of the 

 shock, and similarly with regard to the end. But on this 

 sufficient is said in the questions given below. A rough 

 curve might with advantage be drawn, showing the 

 variation in intensity of the sound, and, if possible, this 

 curve should be combined in the same figure with the 

 curve of intensity of the shock — a dotted curve, say, for 

 the sound, and a continuous curve lor the shock. Such a 

 figure would exhibit at a glance nearly every detail (except 

 the absolute intensity and the time of occurrence) which 

 it is desirable or necessary to observe. 



The order in which the observations should be made 

 and recorded is of great importance. The following is 

 suggested as a convenient one, though the experience or 

 inclination of each observer may lead him to adopt various 

 modifications : — 1. While the earthquake lasts the whole 

 attention should be given to observing the nature of the 

 shock and sound and their relations to one another ; 

 the variations in intensity and character of each. 

 2. Immediately the earthquake is over, take the time (in 

 the following order: fraction of a minute or second, 

 minute, hour), and estimate the interval between the 

 instant when the shock was strongest and the instant of 

 taking the time. 3. Write notes on the nature of the 

 shock and sound, and their relations to one another. 

 i. Estimate the duration of the shock and sound, and the 

 intervals between the beginning of each and the end of 

 each. 5. Record the intensity. 6. Write notes about the 

 position and occupation at the time of the earthquake. 

 List of Questions. 



1. Name of the Place where the earthquake was observed? 



2. Situation of the i Hiscrver : {a) Whether indoors (and 

 on which floor of the house) or in the open air ? (A) How 

 occupied at the moment of the shock ? 



3. Time at which the shock was felt ? 



4. Xtiture (if t/ie S/iock : (a) Was any tremulous motion 

 felt before the principal vibrations, and for how many 

 seconds ? (h) IIow many principal vibrations were felt, 

 and for how many seconds did they last ? (c) Was any 

 tremulous motion felt after the principal vibrations, and 

 for how many seconds ? (</) Did the shock gradually 

 increase in intensity and then gradually die away, or were 

 there two or more maxima or series of vibrations, and, if 

 so, how many were there '? what were the intervals between 

 them? and what was the order of their intensity? (c) Were 

 the principal vibrations strongest near the beginning, 

 middle, or end ? ( /) Was any vertical motion perceptible, 

 and, if so, was the movement first upward and then down- 

 ward, or rice versa .' 



5. Duration of the Shock in seconds, not including that 

 of the accompanying sound. 



