hy¡¤drom¡¤e¡¤ter [ h¨© dr¨®mmətər ] (plural hy¡¤drom¡¤e¡¤ters)
noun
density-determining instrument: a device used to determine the specific gravity, or density, of a liquid, for example, battery acid. It consists typically of a sealed graduated tube containing a weighted bulb.
(MSN Encarta.

Hydrometry (Hydro - water, metry/meter - measure) use the principles of buoyancy or flotation to measure liquid densities.
The simplest device used to measure the degree of flotation is a hydrometer.
(Monash Scientific - see the link, there is also a picture).

It's called p¨¨se liqueurs or l'ar¨¦om¨¨tre de Baum¨¦ in French: "L'ar¨¦om¨¨tre de Baum¨¦ a deux destinations; il sert pour les liquides plus denses que l'eau; on lui donne alors les noms de p¨¨se-sels, p¨¨se-acides, p¨¨se sirops, etc. Lorsqu'il sert pour des liquides moins denses que l'eau, on l'appelle p¨¨se liqueurs, p¨¨se-esprits, p¨¨se lait, etc.", and
"oBaum¨¦ (oB¨¦), named after the French pharmacist, Antoine Baum¨¦, who amongst other creations, developed the "Aerometer Baum¨¦" or a hydrometer, using the density unit oBaum¨¦ to make up a scale of measurements." (http://www.monashscientific.com.au/Baume.htm).






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Note added at 1 hr 31 mins (2004-08-10 05:56:05 GMT)
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erm... the french text is not too legible, but the main idea is: pesalicores (ES)= pese-liqueurs (FR)= Aerometer Baume = hydrometer.
:))