Tolkien also envissioned mechanical dragons that were used to attack Gondolin, but it wasn't put in the Silm. There is some great discussion about mechanical dragons and dragons in general in the new Silm project.
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Yet the lies that Melkor, the mighty and accursed, Morgoth Bauglir, the Power of Terror and of Hate, sowed in the hearts of Elves and Men are a seed that does not die and cannot be destroyed; and ever and anon it sprouts anew, and will bear dark fruit even unto the latest days.
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