What are those round metal structures that look like skeletons for huge water tanks?

Answer:
It turns out they are called 'Gasometers'- do an image search & you'll see heaps of them. Something to do with regulating domestic gas pressure I think. On wikipedia they are called 'Gas holder'
I always wondered what they were too, I saw many in London and some in cities down here in Australia.
First answer by ID1847612260. Last edit by ID1847612260. Question popularity: 1 [recommend question].