Here's the relevant part of the walkthrough for this section:
Study: Enter and talk to Lord Lloyd Bathelwaite. He talks about collecting books for your use concerning the Celts based on your task. He continues on and discusses Stonehenge's history, that it's 1 millennium older than the Druids, 2007 BC, about the connections of the Chichen Itza, Sphinx, Aegean and Stonehenge and the significance of 2012. It is the end of the calendar for the Mayans and how Stonehenge calculated misalignment is important. The others forewarned of the event that will happen in 2012. Like the builders of Stonehenge misaligned the stones but the inner stone circle will come aligned this year -2012. The danger sounds celestial. Have a look around. When done, be on your way. This time use the front door.
Turn left, climb the ladder and read 2 books about the Celts and Alchemy. In one book, note that the measuring tool you took from the shelf in the cellar has the one dram icon. Note also the markings for other weights. Go to the other side of the room. Look at the maps on the table. Place in inventory a map.
Go towards the Grandfather clock. Lord Bathelwaite will give another hint - The important date connected to the Ark of the Covenant is when it was moved to King Solomon's temple. We already know from reading Lord Bathelwhaite's journal in the living room that it is 955 BC.
Clock Puzzle - Look close at the clock. Set the clock to 9:55. Click the minute hand, move it around to 11 and click on it again to release the hand from the cursor. Now, click the hour hand, turn it close to10 and click it to release it from the cursor. The clock moves and reveals a secret staircase.
Maybe read this over and make sure that you haven't missed a step.