Leading lights or Leading marks are a navigation aid used to indicate a safe passage through a hazardous passage when the hazards may not be visible. They are most often found at harbour entrances and consist of two marks which are usually lit at night.
The marks are placed at different heights along the line of the safe course but separated by some distance so that if you stray off the correct course, parallax will make the marks no longer appear vertically one above the other.
In Swallows and Amazons leading marks were used by the Amazons to allow them to enter Wild Cat Island's Secret Harbour quickly and easily. John asks Nancy I wish you'd tell me about the marking of the harbour. He has seen the stump of a tree with a cross on it, and Nancy says the other mark is the fork of a tree with a big patch of bark off it below the fork. John says Is that what the pilot books mean by leading lights and when Nancy asks What are pilot books? John is pleased to find that there were things even Captain Nancy did not know. The concept of leading lights has been explained to the Amazons by Captain Flint last year, before he went bad. Nancy also says Really, of course, they ought to have lanterns on them, for coming in at night. With lanterns on the marks you could come in through the rocks even if it was perfectly dark (SA11).
The next day John takes a hammer and a few nails and puts nails into the marks for hanging a lantern on. That night they enter the harbour in the dark, and John says: This is going to win us the war with the Amazons .... It's the one thing they think we can't do and we can. They think they are safe from us at night (SA12).
So Swallow can enter the harbour at night during the War between the Swallows and Amazons. Titty uses the lights to row the captured Amazon out of the harbour: whatever happened, she must keep the two lights in line, one above another (SA20). But when sailing in the night John realises he is nearly being a duffer, and they moor for the night at a landing stage on an island (SA21). And then, when we saw your lantern ... said Nancy; Lighthouse said Titty, referring to the lantern on the tree stump (SA22).
Notably, the only time that Arthur Ransome directly appears as the author in the Swallows and Amazons series is when he explains the concept of leading lights:
- ...just once he [John] looked around for himself and saw the two lights one above the other like the stop called a colon, which I am going to make: there, like that. — (SA12)
In Pigeon Post, when they are crushing and panning the "gold", they see Squashy Hat's white spots, and Nancy and John realise that they are in a straight line .... pointing exactly HERE .... Like leading lights (PP22). When Dick wonders if the cracks with red dirt that Squashy Hat has hammered at are all the same crack, Arthur Ransome comments that Dick .... had come very near the truth, but none of them knew it, and Dick himself presently threw the idea away (PP28). The "red dirt" is "gosson" or decomposed ore; no good in itself, but a sign of good stuff lower down (PP35)