1896 - 97 .] Mr Thomas Heath on the Calcutta Earthquake. 487 
referred is, of course, the nadir reading of the circle of the 
instrument. In practice this reading is made every observing 
night at the commencement of the night’s work, and again at its 
conclusion. If the night’s work is a long one, an intermediate 
reading should also be made. It has frequently been found that 
two consecutive readings differed from one another by an amount 
which could not be accounted for either by temperature change in 
the instrument or by errors of observation, and it has hitherto 
been found impossible to refer these anomalous readings to any 
known cause. For instance, in looking over a series of observa- 
tions of known stars made by myself some years ago with the 
mural circle at the Calton Hill, for the purpose of re-determining 
the latitude of that Observatory, I find there are two nights in 
which the nadir readings differed by six-tenths and seven-tenths 
of a second of arc respectively. The principal object Prof. 
Copeland had in view in establishing the bifllar pendulum at 
Blackford Hill was not so much the study of earthquakes and the 
phenomena connected therewith, as the possibility of showing tlqat 
the anomalous readings of the nadir point were caused by, or at 
least had some connection with, changes in the direction of the 
vertical of the transit circle, resulting from undulatory movements 
in the earth’s crust such as this pendulum seems admirably adapted 
to show. In the case of my own observations just referred to, the 
latitude results from each star would come into closer agreement if, 
supposing an earth tremor to have taken place, we divide the 
observations into sets and refer them to the separate nadir read- 
ings, instead of using the mean of these readings for all stars alike, 
according to the usual practice. But the pendulum was not then 
established, and, besides, my observations were not sufficiently 
numerous to draw any inference of this kind from them. As yet, 
we have had no opportunity at Blackford Hill of putting this 
question to a practical test. 
Outside the range of astronomy, however, and in a department 
of science which the Society of Psychical Research claims as 
exclusively its own, there would seem to be a possibility that 
instruments very similar to the bifilar pendulum in principle, 
though different in design, might be made use of with considerable 
hope of success. The interesting, if not also amusing, correspond- 
