108 



REPORT — 1892. 



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arran^-ement requires two such pieces of apparatus, which may be placed 

 to work on the same scale as shown in the plan. 



Everything must be well sheltered from currents of air. For a 

 continuous record the glass pointer is replaced by a small galvanometer 

 mirror, which reflects a ray of light through a slit into a box, where it is 



photographed upon a moving plate. 

 The screws B and C serve to adjust 

 the pointei's and to measure the 

 .amount of tilting corresponding to 

 any given displacement. By moving 

 A the degree of stability is adjusted. 

 I adjust ah c, which may be briefly 

 described as an extremely light 

 conical pendulum, until it has a 

 period of about five seconds. With 

 this adjustment the mirror or pointer 

 may at any time be set swinging, 

 and it will return to rest and show 

 the reading from which it was dis- 

 placed. Sources of error which may 

 possibly interfere with the records 

 which these instruments are sup- 

 posed to give are as follow : — 



1. By unequal expansion of dif- 

 fei-ent portions of one of these in- 

 struments the pointer or ray of light 

 might be displaced. As at least four of these instruments, which are 

 differently constructed, some with wire suspensions, and others with 

 fibre suspensions, only vary in amount of displacement, and seldom, if 

 ever, in direction, whilst the temperature is changing, it v.'ould seem that 

 temperature effects are too small to be observed. Lighting a stove in 

 the room and raising the temperature quickly does not appear to pro- 

 duce any effect. Good spirit-levels are subject to great changes by 

 changes in temperature, and therefore it is diificult to use them as 

 recorders of changes in level. I have had a pair of such levels under 

 glass cases, standing on excellent stone columns, for several years, but 

 the results were too unsatisfactory for publication. When they were 

 side by side and parallel the bubble of one might move to the right, and 

 the other to the left. 



2. In consequence of hygroscopic changes the instruments carried by 

 a silk fibre may possibly be affected. I have not observed such changes, 

 but Mr. F. Omori, who is repeating my work, tells me that they are 

 evident. I cannot imagine them to exist in the instruments which have 

 wire suspension, and by the use of quartz fibres which Mr. F. Boys, 

 F.R.S., has sent me, this possible source of error may be eliminated 

 where fibres are employed. 



3. Because the horizontal member of the conical pendulum has in 

 several cases been a fine steel needle, magnetic effects may be introduced. 

 If these are measurable, they must be extremely small. With a 

 8-foot ray of light during a tremor storm, the image may be continuously 

 moving through a range of from 1 to 2 inches. This is too large to be 

 magnetic. If a record for twenty-four hours is repi'esented by a straight 

 line 12 inches in length, I find that this from time to time is gradually 



