War . War never changes . <| sep |> The Grandmaster turns his head to look at the Ambassador . Oh ? Oh yes , our latest advancements . He thinks for a moment before speaking . Well just a few months ago , one of your countrymen developed something rather fascinating . A tube full of mercury from the outside , but the mercury rises as the air pressure rises , and falls when the air pressure falls . It looks like this will be useful as an early warning to detect storms , but we haven ' t thought of all the potential applications yet , I ' m sure . <| sep |> It stopped it again in the deathly hallows , where Voldemort was using the Elder Wand and Harry was using Malfoy ' s . Probably has something to do with Harry being the true master of the Elder Wand .