From: Doug Clow (a fully fledged chemist)
This can and does hold water and is actually used by the Dept as a primary source of water. It ensures a constant pressure head, free from fluctuations in the general supply. A reliable source of water is important in Chemistry - both for providing suction via water-driven vacuum pumps and for cooling purposes. A sudden loss of water supply to most premises is annoying. A sudden loss of water to a place where it is used to keep potentially nasty reactions from getting out of hand is a bit more disconcerting.
As far as I know, about the only cold water taps in the Dept not supplied from the tower are the "drinking water" taps about the place (in the loos and coffee rooms) and (possibly) the toilets.
Structural engineers have been seen (by me and many others) dangling from climbing ropes to inspect the condition of the concrete after the repairs a couple of years ago, when the whole edifice was encased in scaffolding - a sight which would be more impressive if I hadn't been exposed to the even more impressive scaffolding around the Minster.
There's a picture of the tower being built on the front cover of the January 1998 issue of University Magazine.
The Siward's Howe Watertower