The chief portion of this rig is the derrick, Oil which consists of four strong uprights or legs held in Derrick position by ties and braces, and resting on strong wooden sills, which are preferred, as a foundation, to masonry.
The crown of William the Conqueror and his immediate successors seems to have been a plain circlet with four uprights, FIG.
The span to the centres of the end uprights is 820 ft.; width between centres of main uprights at bed-plate 100 ft., and between centres of main members at end of centilevers 20 ft.
The shelves should be of slate a, a, supported by iron uprights b, b, each half having a front ledge of bricks set on edge in cement c, c. The slabs of slate forming the shelves should not be too closely fitted, as a small interval will prevent the accumulation of moisture at the bottom of the bed.
This axis is a hollow forging of nickel steel, of which the accurately turned pivots rest on bearings attached to cast-iron uprights bolted upon a massive cast-iron base plate.
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