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If the hole jewel be of the proper shape, the end-stone not too far from the hole jewel and too much oil is not applied at one time, the oil will not spread nor run down the staff, but a small portion will be retained at the acting surface of pivot and jewel, and this supply will be gradually fed to these parts from the reservoir between the jewel and end-stone, by the action of capillarity.
When a drop of oil is placed between two surfaces, both of which are convex, or one convex and the other plain, as shown at g, it will collect at the point n, at which the surfaces nearest approach one another. We now see very clearly why the hole jewel is made convex on the side towards the end-stone and concave on the side towards the pivot.
Here the chamber is an accurate rectangle, and the portico is formed by adding two side-slabs outside one of the end-slabs, but still under the cover. This last is a remarkable block of limestone weighing about 70 tons. This form of tomb is without doubt a link between the simple dolmen and the corridor-tomb. The portico was at first built under the slab by pushing an end-stone inwards.
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