533 
covered with a layer of cement, so that it trembles when walked 
upon. Yet I succeeded in constructing a fairly stable mounting; near 
one of the corners of the roof on two walls that crossed each other 
and were raised a few centimeters over the roof, two heavy beams 
were laid and cemented down and on the top of these two thin beams 
were nailed on which the tripod of the instrument could be placed. 
For further illustration see the picture on plate I. This shows that 
the tripod was made heavier by a big block of stone and we took 
care not to touch the supporting beams, although we had to adopt 
rather uncomfortable poses for some of the positions of the telescope. 
We soon got used to this, however, and the end was attained. We 
could now walk round the instrument, even stamping our feet, without 
causing the bubble of the level to move in the least. Afterwards 
Mr. Sarim arranged his station as Mecca in exactly the same way 
on the roof of a house rented by me. 
The only thing sometimes preventing pleasant and quiet working 
was the noise in our neighbourhood. Regularly every evening at 
about 8 o'clock there was a musical performance by the Turkish 
military band at Jidda, and even more troublesome was the noise often 
occurring in the evenings in my neighbour’s house and occasioned 
by an ice-machine making almost two turns per second. All we 
could do was to wait till quiet should return, although sometimes 
stars were lost in this way. 
We had also made a point of determining, if possible every night, 
the zenithpoint of the instrument on a signal at some distance. This 
was done in order to continually control the mutual stability of the 
parts of the instrument, and also to facilitate the computation of the 
observations and to trace immediately eventual errors, e.g. in the 
reading of full degrees or in the employed star. At Jidda we used 
as signal a lantern with a circular hole placed on the roof of the 
sufficiently far off French consulate. At Mecca the observer used a 
black spot on the wall of a post situated on the Jebel Abu Kobeis 
a hill quite close to the town. 
3. Value of the parts of the level. Zenithpoint of the instrument. 
The divisions of the alidade-level are numbered in such a way 
that if the reading of the bubble is too low, the reading of the 
verniers must be increased. The value of a division was measured 
a couple of times by displacing the alidade, the instrument being 
clamped. 
