Most people are familiar with the simple stem and bulb thermometer, which depends on the fact that liquids expand when heated and contract when cooled. However, there is an alternative arrangement called the Galilean Thermometer, invented by the same Galileo Galelei (1564-1642) who was so instrumental in the history of astronomy. Various sources give the year of invention as either 1593 or 1608, but either way it was quite a while ago.

You see, the liquid always weighs the same amount. Thus, when it gets warmer and expands, its density goes down. When it gets cooler and contracts, its density goes up. Thus, if you take a column of liquid and arrange in it a series of weights of slightly different densities, more and more of them will sink as the liquid warms and its density decreases. As its density reaches that of each succeeding weight, that weight will sink.

Nowadays you can find fancy Galilean thermometers made of blown glass, with the different weights being little glass bubbles filled with colored liquids, in most shops catering to the "scientific toy" trade. They're not very portable, but they are quite pretty.