Jerusalem

I have heard that Blake was referring to pre-christian monuments like Stonehenge, which were generally thought to be a place where dark pagan rites and human sacrifices were performed, and whose circles of stones were "mills" like the milled edge of coins.

I sing 'those' because I grew up surrounded by tdsm, my parents, aunts, uncles, grandparents had all worked in them. I interviewed my paternal grannie and her workmates in the weaving shed when I was about 13. Never been so overwhelmed in my life, all these large women in those wrap round pinnies, full of life and laughter and surrounded by huge, noisy machines,with the air full (it seemed to me) of wisps of cotton or fluff.
I know it should be 'these' but somehow, emotion gets me every time.

I always sing 'these' because that's what's printed in the Hymn Book!

I was told, many years ago (and have no idea as to how accurate this is) that it referred to workshops and offices that made city dwellers work hard in punishing conditions.
We imagine that hard working conditions came about during the Industrial Revolution, but there were a lot of workshops before that, and apparently office work - constant copying by poor light, peering and bending - was also very bad for health. Most of these buildings were poorly lit.
I'd love to know if anyone has any other explanation.

Yes, sally it's because Blake either lived, or spent a lot of time in Cornwall, and there is a legend that Jesus travelled with his merchant uncle, Joseph of Arithamea, to the region where tin was mined , and this is what Blake had in mind ( the tin mines.....dark, satanic mills.)