of the Acceleration of Gravity for Tokio, Japan, 51 



large enough for electric registration; and the total number of 

 a long series of swings could, with an accuracy of a hun- 

 dredth of a second, be ascertained by the second chronograph 

 method we employed. It would take too long and trespass 

 too much on your space to compare the advantages derivable 

 from this method of getting the time of swing, using a ship's 

 chronometer instead of the older method of coincidences, in 

 which a clock was employed. 



That we are not alone in finding difficulties in the use of 

 the reversible pendulum, and thai: therefore some superior in- 

 strument is not undesirable, is seen from Major Herschel's own 

 artic'e in ' Nature/ where, in spite of his present disparaging 

 remarks regarding our preference for our long wire pendulum, 

 he says : — " The recently recognized fac!: that most, if not all, 

 the modern absolute determinations with a reversion-pendulum 

 are vitiated to a sensible but unknown extent, which can at 

 best be approximately estimated, is noticed " in Colonel 

 Clarke's ' Geodesy;' and he admits that that writer was not 

 unreasonable in excluding " the absolute determinations, both 

 those anterior to Baily's time and those of more recent date." 



So anxious are we to consider every possible cause of error 

 in our experiments, that we take this opportunity of mention- 

 ing two others not alluded to in our paper — the one the earth's 

 magnetic action on the pendulum-wire during the short time 

 in each swing that the recording electric current was passing 

 along it, the other the heating of the wire by this current and 

 the consequent elongation of the pendulum. It will be seen 

 at once that the first effect will be to produce a very slight 

 horizontal force acting in a fixed direction for the twenty-fifth 

 of a second or so each time the wire at the bottom of the pen- 

 dulum-bob was in the mercury, the effect of which will be to 

 alter excessively little the direction, but not the amount, of the 

 force of gravity. As regards the second effect, since the re- 

 sistance of the 934*99 centimetres length of steel wire 045 mil- 

 limetres in diameter could not be more than 8 ohms ; and 

 as the rest of the circuit, including the electro-magnet of the 

 recording instrument, was not less than 10 ohms, the heat de- 

 veloped in the wire while the current was passing, even 

 assuming no heat to be lost by radiation and convection 

 while the current was each time passing, could not, with 

 three Daniell's cells, have been sufficient to lengthen the wire 

 by the hundredth of a millimetre; and as the wire would be 

 thoroughly cooled between each passage of the current, no 

 possible error could be introduced from an accumulation of 

 heat. 



As regards any error which it might be suggested could 



E2 



