Mjdtexan,
Specific gravity is a measure of the density of a liquid compared to water. If you use your hydrometer on distilled water, it should read 1.000, meaning the same as water based on the definition in the last sentence.
For our purposes as winemakers, we start with some juice that is made of water, wine solids, and sugar. We add yeast, and that yeast leaves the water and wine solids but eats up the sugar and spits out alcohol. Since the water and wine solids are staying the same, the change in measurement of specific gravity (the density of the fermenting liquid in compared to plain water) is showing the sugar being eaten by the yeast and the alcohol levels rising. Based on the starting and ending specific gravity, we can calculate the % of alcohol in the wine as well. So the specific gravity isn't as important as the change in specific gravity, which shows us that the sugar has been consumed.
Appleman posted a chart a ways back that showed the correlation between ending specific gravity and residual sugar - this is a useful chart if you are going to be sweetening your wines after fermentation or if a wine did not ferment to dryness.
- Jim
