SOME AUSTRALIAN EARTHQUAKES. 417 



Professor Milne wrote of it : — " I found . . . much to admire. In 

 some directions the instrument embodies improvements on its prede- 

 cessors. I have examined two sheets of recoi-ds it has given, and on 

 one I find a hirge earthquake, the first, w far as I know, automatically 

 rtcorded in London." 



Our seismograph has, tlieiefore, something of historic interest 

 attaching to it A brief description may be here not out of place : — 



The seismograph consists essentially of two parts, (1) that which 

 is sensitive to the earthquake waves, (2) that which i-ecords these 

 waves. ■ 



The part which, so to say, '' feels" the earthquakes is a light 

 aluminium boom or horizontal pendulum, suspended from a short steel 

 upright bar, and swinging to and fro on a pivot at the bottom of this 

 bar.^ The bar itself is part of an iron stand which rests on a brick 

 and concrete pier going down some distance into the foundations of 

 the building, and quite separate from the floor, as shown in Diagrams 

 1 and 2. 



The boom is balanced by a weighted cross-bar, and supported by 

 a thin silken tie to prevent sagging. Its free end bears a lozenge- 

 shaped jdate, shown in Diagram 3, in which is a slit which is just 

 above and at right angles to a slit in the box covering a revolving- 

 drum — the recording part of the seismograph. Upon the intersection 

 of these two slits light from a small lamp is reflected by a miiTor, as 

 seen in Diagram 1, and makes a straight line when the pendulum is 

 at rest, and a wavy line when it moves (as it does when an eaii:hquako 

 wave reaches the pier) upon the photographic paper on the drtim. 

 Sudden and short, deflections are, however, occasionally caused by the 

 presence in the box of an " undesirable immigi-ant," such as a spider.. 



The recording ari-angement in the Adelaide seismograph is a new 

 and improved ^ne. The light cylinder is mounted on a steel spindle,- 

 one of the projecting ends of which has a deep-threaded spiral, on 

 whicli the drum advances 6 millimetres for one turn in four hoiu-s by 

 gear connection with a clock, so that the bromide paper on the 

 dium need only be changed once in four days. Another advantage is 

 that only half the quantity of paper required for the original recorder 

 is needed for this one; moreover it is easier to inspect and store the 

 records, to recogni.se slight tremors, and to measure wave periods. 



The time is marked hourly by an electro-magnet, connected with 

 the clock, moving a shutter on its armature across the slit in the 

 box for a period of a few seconds. 



Some of the ends in view in the installation of the seismograph 

 \uay be told in Professor Milne's own words : — " To determine the 

 \eiocities in which motion is propagated round and possibly throuo-h 

 the earth ; to deterinine the foci of submarine disturbances, such as 

 tliose which have interfered with ocean cables; and to throw new 

 light on changes taking place in ocean beds."' 



It is to be hope<:l that the Australian contribution to this research 

 may be a worthy one. 



The photographs of the Adelaide seismograpli which accompany 

 this paper Avill seiTe to illustrate the description given. 



Appended herewith is a list of earthquakes in South Australia 

 since November, 190-3, the last date for which the records have been 



2b 



