So,
we need first to define what we mean by "achievement".
e.g.
any artificial construct
any artificial construct that serves a useful purpose
any artificial construct that serves its purpose and advances engineering
and so on.
We might incude the pyramids because they fire our imagination (party because they seem so ambitious a project compared with our perception of the skills available) and the question of the extent to which slaves were employed is debated....some suggest that in a river culture with wealth, and a large permanent population with largely seasonal employment that it is a bit like the CCC in America. But let's not go there.
We might also disqualify projects such as the pyramids and the great wall of China on the grounds that as engineering achievements they failed.... or that they are still here long after their useful purpose was served (unless you count tourism, which surely wasn't in the design specifications). Imagine if 4000 years from now all our engineering achievments are still cluttering up the planet.
So we might consider that we are looking for something that has endured. The wheel surely fits that bill and fire is a discocvery, but while the bow drill might be legitimate, who uses it any more. Actually, since we don't have a planet full of useless bow drills, it might be valid. Fint and steel are certainly still available and used.
Size and age surely aren't sole criteria. Velcro could prove pretty enduring. After all, the principal is an enduring part of nature. (copying from nature is OK by me).
I personally don't think mathematical ability or draftsmanship are necessary qualifications. It is the mind that is the important tool.
We might consider earthquake proof architecture.... in which case the Chinese can lay a claim.
Perhaps we should include a "before their time category" for successful engineering projects. By this I mean those projects that we tend to think of as fitting within a particular technological era. For example, drilling for natural gas... again we give the prize to the Chinese for their gas wells in 970BC (i think, but in this comment on salt production from evaporation we have at least 200BC:
Thoughts?
If not I'll put forward my original suggestion temporarily which is the wooden wedge, invaluable in quarrying the marble for the great sculpturers. When driven dry into a crack in the stone and then soaked in water it employs hydraulics and not brute strength for its action.
JMW
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