August 7» 1890] 



NATURE 



347 



study in this country would be a system for securing 

 accurate time-records whenever a shock is felt. 



Seismoscopes. — A large number of simple and inex- 

 pensive seismoscopes have been devised and used for 

 recording the time of occurrence of an earthquake shock ; 

 but it is difficult to find one that in all respects is 

 thoroughly satisfactory.^ To be so, they should fulfil the 

 following conditions :— They should be inexpensive, 

 simple in their construction, easy to arrange, and require 

 little attention when once erected. They should record 

 the occurrence of shocks and tremors with equal facility 

 in whatever direction they may arrive ; and they should 

 be equally sensible in recording a feeble shock. It is 

 very desirable also that they should be of similar con- 

 struction, at any rate in a given district, if not throughout 

 the whole country, so that observations from different 

 places may be rightly comparable. 



Again, in countries where earthquakes are frequent, 

 and where the shocks may succeed one another at short 

 intervals, it is important that the record should be made 

 without stopping the clock. In Great Britain, however, 

 our catalogues show that, except at Comrie, it is not 

 usual for sensible earthquakes to follow one another 

 rapidly, and it is therefore worth while considering 

 whether, on account of their much greater cheapness and 

 simplicity, it might not be well to avail ourselves in this 

 country of clock-stopping apparatus. Such instruments, 

 are, of course, defective in that, until re-set, they are 



incapable of recording a second earthquake. But they 

 possess a compensating advantage in the accuracy with 

 which it is possible to time the occurrence of a shock. 



Clock-stopping Apparatus. — As it is possible to make 

 with ease, and at little cost, an extremely delicate appa- 

 ratus for stopping a clock at the moment of a shock, I 

 quote the following description of one devised by Prof. J. 

 Milne (Japan Seism. Soc. Trans., vol. iii. pp. 61-62) : — 

 " P is the pendulum of a clock with a small piece of 

 wire standing out at right angles to its face. . . . This 

 wire, as the pendulum swings, passes beneath a series of 

 teeth cut in a strip of wood lightly hinged at A and 

 terminating at the other end, B, with a piece of stiff wire. 

 ... If such a contrivance is allowed to fall, the teeth 

 catch on the projecting pin of the pendulum, and it may 

 arrest it at any portion of its swing." The arrangement 

 which, at the time of an earthquake, allows the toothed 

 lever to fall " consists of a piece of stiff wire, w, on which, 

 near to one end, is a small cylinder of lead, L. The short 

 end of this wire is pointed, and rests on a pivot-hole 

 made in the head of a drawing-pin pressed into the side 

 of the clock-case, S. To prevent this wire from falling, it 

 is held up by a small silk thread, T, fastened to a second 

 drawing-pin. As suspended, it is very unstable, and, 

 instead of remaining at right Angles to the clock-case, it 

 swings round against it. When, however, the wire, B, 

 rests on the end of w, it retains its position, as shown in 

 the figure." 



This instrument is so sensitive that it is difficult even 

 to shut the clock-case without stopping the clock. The 

 reason of this appears to be " that, if the clock-case 

 receives a small displacement at right angles to w, the 

 weight remains steady by its inertia, whilst the long arm 

 of w in contact with B multiplies the initial motion" 

 approximately in the proportion of the length of the long 

 to that of the short arm of w. 



It would appear that a displacement parallel to the 

 wire W would not give this multiplication ; but, practi- 

 cally, Mr. Milne observes, " it seems impossible to give 

 a motion in that direction to which the apparatus does 

 not seern to be just as sensible as to a motion in any 

 other direction. The only other motion which does not 

 result in stopping the clocks appears to be a very sloxu 

 easy swing ; " and thus the instrument will probably be 

 incapable of recording the occurrence of the dying-out 

 vibrations of a very distant shock. 



The instrument may be placed in a cellar or out-house, 

 or out of doors under the cover of a close-fitting box. A 

 strong stake should be driven into the ground, to the 

 depth of two or three feet, the floor, if any, being removed 

 for a few inches round the stake to prevent the instrument 

 being disturbed by the vibrations of the house. The 

 clock-case should then be screwed firmly to the stake. 



If several of these instruments are erected in a district, 



' A Committee of the British Association is at present considering the 

 form of seisnioscope most suitable for use in this country. 



NO. 1084, VOL. 42] 



they should be placed at distances of not less than 5 to 10 

 miles apart.^ The sites selected should, if possible, be 

 free from the vibrations of passing carts and trains. If 

 two or three of these record the occurrence of a shock 

 at very nearly the same instant, it may be inferred that 

 the disturbance is not accidental in its origin, and the 



I inference will be strengthened if several instruments 

 closely agree in their indications. But a record from one 

 alone must obviously be regarded as doubtful, if all the 



I others were at the time in good working order. 



Suggestions for the Observation of Earthquakes {with- 



! out the use of special instruments).— Lists of questions. 



j for aid in the study of earthquakes have been drawn up 

 by Prof. Heim and Prof. Milne.- The following ques- 

 tions are founded partly on these lists, but chiefly on the 

 accounts of earthquakes in different places, and especially 

 in this country. It is hardly necessary to insist that all 

 notes should be written down on the spot, or as soon 

 after the shock as possible ; but it may be useful to re- 

 mark that it is often just as important to note when a 

 given phenomenon is not observed as to describe it fully 



' If the clock be carefully rated, it should be possible to obtain the time 

 of a shock correct to a tenth of a minute. The velocity of earth-waves is 

 subject to wide variations, even in traversing the same rocks ; but, taking it 

 at 1000 feet per second, it follows that the earth-wave will pass over more 

 than a mile in one-tenth of a minute. A good deal more than a mile, then, 

 should separate every pair of stations where seismoscopes are placed. 



" A. Heim, "Die Erdbeben und deren Beobachiung " ; Arck. des Sc. 

 phys. et nat., ync per. t. iii. pp. 286-7 : Fouque, " Les Tremblements da 

 Terre," pp. 133-4", Japan Seism. Soc. Trans., vol. i. part ii. pp. 3-4. 



