| 摘要 |
A parallel rule comprising two rulers 1, 2 linked by at least two equal-length parallel cross-arms 3, 4 which are rotatably mounted onto the rulers 1, 2 by simple loose rivets or machine screws 10. Three of the mountings allow free rotation while the fourth consists of a tightenable and lockable system comprising a blind knurled nut 5 and two friction washers 8, 9. The screwing down of the knurled nut 5 locks the rotation of the cross-arm 4 into the required position. The parallel rule is of particular utility as a navigational aid for seafaring vessels to plot their position onto a chart when this position has already been determined by a global positioning satellite.
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