A barometer is an instrument that measures atmospheric pressure. The barometer measures pressure exerted by the atmosphere using water, air, or mercury. Through measuring the pressure one can forecast short term weather changes.
Evangelista Torricelli is credited with the invention of the barometer in 1643. However, Gasparo Berti unintentionally built a water barometer around the same time. Rene Descartes was also noted as describing an experiment to determine air pressure but there is no record of him building a working barometer.
A simple version of Berti’s experiment consisted of filling a long tube with water that had both ends plugged, then standing the tube in a basin already full of water. The bottom end of the tube was opened, and water that had been inside of it poured out into the basin. However, only part of the water in the tube flowed out, and the level of the water inside the tube stayed at an exact level, which happened to be thirty-four feet, the same height Baliani and Galileo had observed that was limited by the siphon. The important part was that the lowering water left a space above it which had no contact with air to fill it up. Seemingly this suggests the possibility of a vacuum existing in the space above the water.
Traditionally it was thought that the air did not have lateral weight, meaning the air above us put no weight on us. Galileo believed the weightlessness of air to be true. Since Torricelli questioned that belief and found the barometer as an instrument for measurement, instead of a device to create a vacuum, he is generally considered the inventor. Torricelli eventually reproduced his measurements with mercury.