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been done at the observatory at Leyden under the supervision of 
Prof. van DE SANDE BAKHUYZEN, who also investigated the best methods 
of combining the observations. The extensive calculations have been 
for far the greater part executed by Mr. H. W. Hamersma, late chief 
mate of the Royal Dutch Navy. 
In this way results have been obtained for the geographical position 
of Mecea which certainly exceed in accuracy all that has been 
known up to this time and therefore I take the liberty to offer a 
paper on this subject to the Royal Academy of Sciences. 
II. /nstruments. Stations of observation. 
The instrument with which our observations were made was the 
universal-instrument Pistor and Martins N°. 905, belonging to the 
Technical University at Delft and kindly lent to me by Prof. Hevverink. 
The same instrument had formerly been employed by Mr. S. P. 
L’Honoré Naser of the Royal Dutch Navy for his observations for the 
demarcation between the republic of Liberia and French Congo. 
The telescope of the instrument is at the end of the horizontal axis, 
while for the observation of small zenith-distances a reflecting prism 
ean be brought before the ocular. The circles are read with verniers, 
the diameter of the vertical circle is 130 mm. and that of the 
objective is 28 mm. The value of a division of the level attached 
to the alidade-circle is about 8". During my first stay at Leyden the 
spider-lines were broken. They were replaced by two horizontal 
threads only, at about 6’ distance from each other. 
For a moment we had thought of employing instead the small 
universal instrument of the Leyden observatory, which has the same 
dimensions but is read with micrometer-microscopes. The consideration, 
however, that it is advisable in the damp and warm climate of Jidda to 
make as little use as possible of spider-threads especially of movable 
ones, made us give up this idea. 
In the choice of the chronometers to be used, particular attention 
had to be paid to the peculiar circumstances attending the transport 
from Jidda to Mecca, which is done by camels, so that shocks cannot 
be altogether avoided. Moreover the road is far from safe; nearly 
every year a caravan is attacked and robbed by Beduins and it is 
therefore desirable not to take any conspicuous boxes. To carry these 
on foot would be altogether impossible. Discussing this point with 
Prof. VAN DE SANDE BAKHUYZEN and Mr. C. F. J. Cosyn, Chief of the 
bureau of instruments of the Royal Navy, the latter drew our atten- 
tion to the pocket-chronometers of Leroy, the so-called chronomêtres- 
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