This is a watch produced during the 1940s by Jaeger (the EU version of what is now Jaeger-LeCoultre, which produced watches for the US under the name LeCoultre). These were used as desk clocks for offices or a portable clock for travellers (that's why the temperature function really is useful). The specific watch Dennis purchased I believe it to be made of brass with a rarer case shape which is not usually seen. As you can see, the watch has three main sections: a time and alarm clock with an 8-day power reserve, a thermometer and a barometer, which has an inner pressure chamber which helps "predict" in the old-fashioned way the weather. Additionally, on the inside of the barometer dial you can see a smaller arrow (which is manually actioned to benchmark the current pressure and observe any changes when compared to the bigger arrow hand) that can be actuated by rotating the red button-like part that you can find on the right side of the clock.