December, 1905.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



295 



Seismoscopes. 



Bv Charles Da\ isox. ScIJ., l'\(i.S. 



Instruments desisfneci for the reg-istration of earth- 

 quakes may be divided into three classes : (i) those 

 which give the time of occurrence only; (2) those which 

 record and magnify the actual movement of the ground 

 without any determination of time or duration, and {3) 

 those which register the time, amplitude and period of 

 every vibration, so that the actual movement of the 

 ground may be completely realised. Those belonging 

 to the first class are usuallv known as seismoscopes, and 

 those of the second and third classes as selsntograp/is. 

 The latter as a rule are elaborate and costly apparatus, 

 and can only be constructed by skilled workers and 

 with the aid of refined tools. On the other hand, some 

 forms of seismoscopes may be easily and cheaply made, 

 and the errors due to home-manufacture are not of 

 much consequence. They establish, what is an impor- 

 tant fact, that a disturbance of some kind has taken 

 place, and, without much trouble, they may he made 

 to record its time of occurrence. 



In a paper published in " Knowledge" for .August, 

 1896, a few suggestions were offered for the observa- 

 tion of earthquakes without instrumental aid. The 

 present paper, which may be regardv'd as a supplement 

 to the other, contains descriptions of a few of the sim- 

 plest kinds of time-recording seismoscopes. 



Every such -,seismo.scope consists of two distinct 

 parts, one for magnifying the movements of the 

 ground, and the other for recording the time. In the 

 latter part, there is some, though not g^reat, varietv, 

 seismoscopes, otherwise different, making use of the 

 same kind of recorder. .\ good clock, or one the error 

 of which is known, is its essential feature. The 

 simplest tvpe of seismoscope is one in which some ar- 

 r.-ingement is made for stopping a clock, or starting 

 a clock (previously fixed at some known time), at the 

 instant when an earthquake occurs and attains suffi- 

 cient streng^th to affect the instrument. In the best 

 form of all, the record is made without stopping or in 

 any wav affecting the motion of the clock, and it is ob- 

 vious that, in a country where earthquakes are at all 

 frequent, this kind should be adopted whenever 

 possible. Even in Great Britain, nianv records may be 

 lost if the instrument is put out of action bv the first 

 tremor of a series. For instance, within seven hours 

 on the night of December 16-17, 1896. at least ten 

 earthquake-shocks were felt in the neighbourhood of 

 Hereford ; while, during a still shorter interval in the 

 early morning of September 18, 1901, one observer near 

 Inverness counted no fewer than twenty shocks. 



Mallet's Seismoscope. 



One of the earliest and simplest seismoscopes 

 adapted for stopping clocks was that devised by the 

 well-known seismologist, Robert Mallet. This is shown 

 in fig. I, in which .\ represents the bob of the clock 

 pendulum, B a piece of stout wire passing through the 

 centre of the bob at right angles to the plane in which 

 the pendulum oscillates, and CD a strip of wood, an 

 inch or an inch-and-a-half wide and a quarter-of-an- 

 inch thick, weighted at the end C, and turning freely 

 about a pin driven into the wall or some steady support 

 at the end D. This lath passes through holes in the 

 .side of the clock-case, and its lower edge is cut into 

 teeth in that portion which covers the arc of oscilla- 



tion of the pendulum. E is a log of heavy wood, four 

 to fi\e feet in height and fi\e or six inches square ; the 

 lower end, which rests on the ground, is cut off square, 

 and the top is cut down to about a quarter-of-an-inch 

 square, so tliat the l:ith (' I) iiiav rest upon it. W'hrn 



Fig. 



-Mallet's Seismoscope. 



the log E is o\erthr()wn by a shock, the lath C D falls, 

 and the teeth, catching the pin B, will stop the clock.* 

 The dimensions of the apparatus may obviously be 

 varied at pleasure ; its chief defect lies in the roughness 

 of the starter E. 



Milne's Seismoscope. 



Professor Milne's seismoscope is free from this de- 

 fect, in having a more sensitive arrangement for 

 dropping the lath C D. Instead of the column E 

 (fig. I.), he uses a simple horizontal pendulum EG C 

 (fig. 2.), which is more readily displaced by a weak 

 •:hock. The lath C [■) and the pendulum bob are ar- 

 ranged as by Mallet, but the lath ends at C in a piece 

 of wire. E C is a straight wire, passing through a 



u 



Fig. 



disc of lead 1-". The end I'- is sharpened .ind rests in a 

 small conical hole m.ade in the head of a dr;iwing-pin 

 pressed into the side of the clock-case, and the wire 

 E C is supported in a horizontal position by a silk 

 thread G fastened to the clock-caAe at the upper end by 



•In another form of seismoscope suggested by Mallet, two strings 

 pass from the log of wood through the sides of the clock-case, and 

 are fastened to the pin at the lower end of the pendulum. The 

 lengths of the strings are adapted so that, while hanging loose 

 within the clock case, they permit the pendulum to swing Ireely ; 

 but stop it immediatly the log E is thrown down by a shock 



