The Shutter Tower

The line

In 1822 the Admiralty built a chain of shutter telegraph stations from the Royal Naval College at Greenwich to the dockyard at Portsmouth. Each station was a tower with six rectangular shutters mounted on a frame, arranged in two columns of three. Each shutter could be open or closed, which gives 64 possible combinations, and each combination stood for a letter, a numeral, or a control signal such as "message received" or "repeat".

The line carried a message from London to Portsmouth in about fifteen minutes in good visibility, which was roughly ten times faster than a horse. It was made obsolete within thirty years by the electric telegraph, and almost every tower was demolished. The one you are standing at, on Chatley Heath, is the last survivor with its semaphore frame reconstructed.

The code

The puzzle uses the standard Admiralty shutter code as it was published in the 1822 standing orders. The six shutters were numbered down the left column first, then down the right:

A letter is encoded by opening a set of shutters. The listing gives you six encoded strings, each six characters long, using O for open and C for closed. Decode each one to a letter. The six letters are the first six letters of a word that describes what the tower was for, and the position of each letter in that word gives you a digit.

Do not look for an encoding table on site; the tower's own display boards give the history but not the code. The code is in the listing and it is the standard one.

The arithmetic

You now have six digits, one per letter. Take them in the order the encoded strings appear in the listing and treat them as a bearing in tens of degrees and a distance in metres, split three and three:

• Digits one to three, read as a three-digit number of degrees.
• Digits four to six, read as a three-digit number of metres.

Apply the bearing from the published coordinate and walk the distance. That places you within about fifteen metres of the final. The terrain here is heathland with sandy paths, so the walking is easy and the navigation is the hard part.

The find

The final is a small clip-lock under the parapet of the tower, on the north-east face, behind the second brick up from the string course. The brick is a loose one and comes out with a fingernail. This building is a scheduled monument and is inspected, so put it back flush and do not disturb anything else.

Access and terrain

• The tower stands on Chatley Heath, which is open heathland managed by the county wildlife trust. It is a scheduled monument; do not climb the tower or the frame.
• The paths are sandy and drain well, which makes this a good winter cache. There are no significant gradients.
• Adders are present on the heath. They are shy and will move away if given the chance; do not put your hands into vegetation without looking.
• The heath is grazed by cattle in some years and by sheep in others. Close gates.

After the find

The Wey Navigation towpath is half a mile east and gives a flat walk along a canal built in 1653, with the remains of the original locks and a working weir. It is a good way to finish a short day.