tliat the entire time of su-;[>ensioii was at least two months. As only one 

 raeasurement is spoken of, it is probable that it was raeasureil at the conclusion 

 of the series of experiments, and it seems hardly likely that its length wonld 

 have remained coustant during that length of time. In getting the time of 

 vibration the lirst method used was what might be termed the metliod of coin- 

 cidences by electricity, and which, so far as I kno«'. was first described by 

 Professor Pickering, in his e.vcellent ■"Physical Manipulations." 'i'his was after- 

 ward rejected, however, and the vibrations were counted by means of a Morse 

 instrument. The authoi^s speak of measuring the fraction of a viliration, but 

 evidently this could not be done with accuracy by the iLse of such an arrangement, 

 and there is also the objection that the pendulum was obliged to do the work of 

 breaking the circuit at every vibration. ^lessrs. Ayrton and Perry give the 

 time of vibration of their pendulum for only three ex[)eriiiients, and it is a little 

 difficult to luulei-stand exactly how these were obttiined. The time, taken from 

 the chronometer, is given and also the number of vibrations. The only way to 

 make these consistant with each other is to assume an extraordinary and rapidly- 

 fluctuating clock rate and even then it is impossible to deduce the periodic time 

 wliich tliey use in their calcidations which is considerably greater than that of 

 either of the three experiments given. In applying the air correction they have 

 failed to take account the air dragged by the pendulum in its motion although 

 in a subsequent paper in the same journal they have applied this correction. 

 They liave also omitted to correct for the arc of vibration although this was 

 considerable in tlieir experiments. In cor:sideration of tliese facts it does not 

 seem that the result wliich tliey obtained is entitled to great weight, notwith- 

 standing its cb.se agreement with the calculated value for this latitude when the 

 .proper corrections are introduced. 



