Hobson's Brook is the name of a watercourse, built in the early 1600's by Thomas Hobson of "Hobson's Choice" fame, to carry fresh water into Cambridge. The source of the water was Nine Wells a few miles to the south, where you will find an Earthcache.
If you walk from this cache northwards towards the Fitzwilliam Museum, you will see the conduits that were constructed on either side of the road to carry the water from the Brook further into the centre of Cambridge.
You are looking for a magnetic container and you will need items with which to extract and sign the log.