Leveling the Support Flat
Next it is necessary to level the flat as accurately as possible. I have found that the use of a high-accuracy circular bubble level (a so-called "bull's-eye level") is quite effective. The ones I have been able to source have a resolution of 5' per 2 mm. The L-shaped support members are mounted to the parallel bars by being clamped between nuts threaded upon 12mm brass bolts. The photo below (unfortunately rather blurred) shows the arrangement. The holes through the L-shaped supports are in fact 1/2", which is a bit bigger than 12mm, so as to allow a slight looseness. To raise or lower any of the three support points, you have to wind the two nuts up, or down. This construction works perfectly. The next photo shows that the end of the bar in the cross direction needs to be adjusted quite a bit - about 1/2 turn of the nuts. Now the bubble is virtually in the middle, so the adjustment is correct. At this point, the first time, in order to check your level, you should rotate the bull's-eye and make sure that the bubble remains in the middle of the circle all round - some have been known to be off, and then they need to be trimmed up, which is not difficult... A view from on top. You can see that the bubble is off by less than 1 mm. This means that the support flat is horizontal within 2' of arc. This is adequate, considering that Allais found that more than 12' slope made very little difference to the precession of the paraconical pendulum (you can find this Note to the French Academy of Sciences in the Allais translations section of this website). Back to the description of operating the English paraconical pendulums in Sarawak. Back to the website main page.
