This is an aneroid barometer used to predict the weather. It's actual a pressure gauge. It's a bit like one of those party whistles that you blow into and they unwind. What happens inside is that the high pressure air pushes on a curved tube inside which is attached to a needle. This gives a reading for pressure on the scale shown - 1023 millibars on the inner scale. The recent stormy weather is associated with low pressure. Atmospheric conditions drive air away from an area. This causes lower level air to spiral in to fill the gap like water going down a plug hole. Hence the low pressure end of the scale is labelled with winds and rain. High pressure air brings settled conditions. So if you can measure the air pressure you can guess at the likely weather forecast.
