Form of Seismograph. 355 



which have been introduced into later instruments. It was 

 found that when the " conical pendulums " (see below, p. 362) 

 used for actuating the recording indices were, as in that in- 

 strument, made to turn with very little friction, and were 

 adjusted to have a long period of free oscillation, that is to 

 say to have very little positive stability, the lines traced by 

 the recording points gradually broadened to a very incon- 

 venient extent. This rendered good records of small motions 

 impossible after the record-receiver had been in motion for a 

 short time, and introduced a risk that such records might be 

 obliterated after they had been obtained. Such considerations 

 as these led Prof. Milne to abandon the continuous motiou 

 element, and adapt the instrument to the comparatively old 

 method of automatic starting at the time of the earthquake. 

 There are, besides the difficulty experienced due to the 

 broadening of the lines by the recording points, several other 

 important objections to the use of a band of paper of such 

 limited length as that provided by a single turn round a 

 cylinder of moderate dimensions. The record may, for ex- 

 ample, extend more than once round the cylinder ; that is, 

 the earthquake may last longer than the time taken by the 

 cylinder to make a complete turn. This produces great con- 

 fusion in the record, rendering it difficult to interpret. 

 Again, two earthquakes may occur before the record-sheet 

 has been changed ; and in such a case both records are practi- 

 cally lost. Considerations such as these have led us to adopt 

 one or other of the forms of apparatus described in this paper. 

 The new form of apparatus has for its object the determi- 

 nation of the same elements as have been already enumerated 

 with reference to the old instrument. Provision is, however, 

 now made for the whole of the record being obtained on fresh 

 surface, and for any number of earthquakes which may occur 

 within a limited period, say a week, being recorded on the 

 same sheet. The record-receiver is kept continuously in 

 motion at a very slow rate, and time is marked on it at 

 regular intervals by means of a good clock; the object being 

 to secure with perfect certainty that most important element 

 in earthquake investigation — the time of occurrence of the dis- 

 turbance. In the most complete form of the apparatus the 

 record-receiving surface is a long ribbon of thin paper, which 

 is gradually unwound from a supply drum on to another, 

 which may be called the hauling-off drum, by means of a 

 weight or spring and a train of wheelwork. The speed is 

 rendered uniform by taking the paper in its passage from the 

 one drum to the other round a third drum, which is kept 

 continuously in uniform motion by a train of clockwork and 



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