Rock on a Rope, how to level the lintels

 

In an earlier post on setting out the Stonehenge sarsens I suggested a simple way of establishing a horizontal: https://www.sarsen.org/2014/08/how-to-construct-level-sarsen-circle.html , place two posts either side of a small pool of water (an “Aurochs skin full of water”) and sight across their tops. Adjust the posts so their tops both have the same length of post above the water surface, the line of sight is level and can be transferred around the circle. 

A complementary approach starts from a different but equally basic physical fact: a free-swinging pendulum released from rest reaches essentially the same height on the opposite side of its arc. A tall central timber mast, a long fibre rope, and a heavy stone or timber bob are all materials the Late Neolithic builders already used. Release the bob from one marked position and it will rise to the matching height on the far side; adjust a second post until the bob just reaches it. For more accuracy, you release the pendulum from post A and it marks on post B the end of the first swing. When it comes back to post A the second time, it's going to be just a little bit lower because of air resistance. Halfway between the first and the second mark on post A will be the same as a mark on post B. Subtle and easy to get an accurate reading. Repetition is quick, free and easy so a reliable and repeatable measure can be recorded.  Rotate the plane of swing or shift the mast a short distance and a small set of levelled reference posts can be established around the intended circle. Once three or four reliable points exist, ordinary sighting or taut-string transfer finishes the job.

The two methods form a small family of “gravity levels”:

• Water-pool / skin method – immediate visual reference, excellent for a local datum, but needs a still surface and a reliable container.
• Pendulum method – needs no liquid, generates widely spaced equal-height points from one central mast, and is cheap to repeat when wind deflects the swing.

Neither is attested archaeologically. Both are simply possible with the technology of the time and address the practical problem of creating a consistent horizontal on sloping chalk so that the sarsen uprights could finish with level tops for the continuous lintel ring. And avoid the need for a water filled pig intestine or portable troughs as are often suggested as alternatives.

Pendulum method being tested