3 
experiments made with an invariable pendulum. 
Hall and General Sir Thomas Brisbane ; that of taking a 
mean between the disappearance and re-appearance of the 
disk, has been practised by Mr. Goldingham at Madras, and 
by Captain Sabine. Theoretically, the mean of the disap- 
pearance and re-appearance, would give the true moment at 
which the two pendulums coincided at the lowest part of the 
arc of vibration, were it the object of this problem to deter- 
mine that moment: but it is not: — the experiment being 
strictly comparative ; — and the method of disappearances 
accomplishes all that is sought after, with perfect certainty, 
and with less than half the trouble. It may, however, be 
useful to know, that both methods give identically the same 
results ; that is to say, the number of vibrations of a pendu- 
lum determined by the method of disappearance at one station, 
compared with the number deduced by the same method at 
another, give precisely the same acceleration or retardation as 
that which would result from comparing the number of vibra- 
tions at the first station, ascertained by taking the mean of 
disappearance and re-appearance, with those of the second 
station, ascertained by the same method. The results of the 
experiment contained in the following paper show this very 
obviously, as follows : 
Vibrations by the method of disap- 
pearance alone at 
Greenwich, 86159,368 
Port Bowen, .... 86230,172 
Acceleration by the method ) _ „ 
of disappearance . , j ~ 7 °> °4 
Vibrations by the method of mean of 
disappearance and re-appearance at 
Greenwich, 86159,500 
Port Bowen, .... 86230,313 
Acceleration by the mean 1 
of disappearance and > =: 70,813 
re-appearance . . . j 
