Intriguing question posed by Alan.
The two towers in Scotchie's photo were constructed in 1830 on either side of the entrance to the Inner Basin, later named the Granville Dock, and removed in 1877. The one in the background is a clock tower. The one in the foreground is a compass tower indicating wind direction and the wind vane is visible above the tower. I imagine that this would be quite difficult to see from some directions, particularly if pointing directly towards or away from you. Therefore one assumes that it drove the four indicators on the tower via gearing. The indicators would need to give the wind direction with respect to due north as the relative wind direction would be irrelevant without knowing what bearing it was relative to. Therefore the twelve'o'clock position on the indicators would have to correspond to due north with the remainder of the 360 degrees being marked out in points of the compass. Following installation or any maintenance, the wind vane and all of the four indicators would therefore have to be aligned accordingly. This is all pure guesswork by the way!
Alan advises that there was a clock face with a single hand in the new clock tower on the Prince of Wales which needed to be adjusted with a compass. Accordingly, my initial supposition was that there must be one face of the clock tower which indicated wind direction in the same fashion as above, with the wind vane requiring to be correlated with the hand to give the compass bearing.
However, looking at the two photos below, all four faces are clock faces. That theory is therefore out of the window and I give up.
Further to the above, true wind direction is also generally displayed onboard ship rather than relative wind direction, i.e. relative to the ships head. The equipment used on the DFDS ferries uses a small anemometer head as pictured below rather than one of the older anemometer heads looking like a small aeroplane with the tail acting as a vane and a small propeller to give wind strength. The device shown uses four arms equipped with piezo electric transducers. These are arranged in two pairs, one fore-and-aft and one athwartships. These transmit an ultrasonic signal across the gap. The wind either speeds or slows the passage of the ultrasonic signal between the transducers. The two signals are then integrated in the electronic unit down below to give wind speed and direction. This is then disseminated to digital displays in the Bridge centre and wings.
I mention this because of the preference of the ships masters to have the indicators showing true wind rather than relative wind. One might reasonably surmise that they would prefer relative as this would show them how the wind is acting on the ship with respect to the ships head. However, they all have what amounts to a compass within their brain and instinctively know the ships heading with respect to true north and consequently prefer the displays set to indicate true wind direction.
This would therefore apply even more so to the old sailing ship masters viewing the compass tower, whose whole life was intimately bound up with a knowledge of what the wind was up to.
