Some physics instruments can be quite captivating due to, for instance, their peculiar shape or material. Take this pear-shaped object known as Nicholson’s hydrometer, for example. With the anniversary of the Great Fire of Turku of 1827 taking place on 4 September, we decided to select as the object of the month one of the treasures that survived the blaze and now features in our collection. What is this pear-shaped, streamlined, metallic object known variously as an areometer, a gravimeter, a densimeter and a hydrometer? The names tell us very little about the object itself, so let’s find out more.