137 
M. G. Moll on Magnetic Observations , <Sft\ 
It explains how the brain of A-ke is in all respects a sensorium 
commune to both. That the parasite is therefore only a dupli- 
cate of the principal, is not more difficult to be imagined than a 
supplementary linger or toe. Here, however, our field expands 
into a wilderness, into which it would be unsafe to enter with- 
out a guide. 1 shall therefore resign the task into the hands of 
more adventurous discoverers. 
Macao, 8 th December 1820. 
Art. XX. — History of the Magnetic Observations said to have 
been made at Zwanenburg * . In a Letter to Dr Brewster 
from M. G. Moll, Professor of Mathematics and Natural 
Philosophy in the University of Utrecht, and Member of the 
Royal Institute of the Netherlands, 
Sir, 
Tn the last Number of your very valuable Journal, I find a 
table of observations inserted, on the diurnal variations of the 
magnetic needle, from 1775 to 1780, which are stated to have 
been made at Zwanenburg in Holland , whilst you were unac- 
quainted with the name of the observer, and the nature of his 
apparatus. Give me leave to say, that these observations, which 
are indeed very valuable, were not made at Zwanenburg, but 
in a village at no great distance from that place, called Sparen- 
dam. I am not aware that observations on the magnetic needle 
were ever made at Zwanenburg for any length of time, although 
the barometer and thermometer have been observed there with 
great care ever since 1743. You may find these observations 
in the Transactions of the Haarlem Society ; and a paper has 
been given in on that subject to the Institute of the Netherlands 
by Professor Van Swinden, which is printed in the 1st volume 
of the Memoirs of that Society. Your correspondent Mr Rum- 
ker could not do otherwise than suppose that the magnetic ob- 
servations alluded to were made at Zwanenburg. They are re- 
presented as made there even in the volumes of the Transactions 
of the Haarlem Society, in which they are printed, and from that 
® Bee this Journal } vol. iv. p> 848=. 
